UNIVERSITY  OF  CALIFORNIA 
LOS  ANGELES 


A.M  V 


[UNIVERSITY 


COMPRESSED  AIR 


PRACTICAL  INFORMATION  UPON 


AIR-COMPRESSION  AND  THE  TRANSMISSION  AND 
APPLICATION  OF  COMPRESSED  AIR. 


BY 

FRANK  RICHARDS,  MEM.  A.S.M.E, 


FIRST  EDITION 
,FIETH 


NEW  YORK : 

JOHN    WILEY    &    SONS. 

LONDON:    CHAPMAN    &    HALL,    LIMITED. 

1909. 


463287 


Y 


Copyright,  1895, 

BY 

FRANK  RICHARDS. 


JJulirrt  Drumming  anb  (Companij 

Nnvforfc 


PREFACE. 


IT  is  only  proper  to  say  that  much  of  the  matter  con- 
tained in  the  following  pages  has  appeared  at  intervals  dur- 
ing the  past  two  years  in  the  columns  of  the  American 
"»  Machinist.  The  publication  of  the  articles  referred  to  and 
the  remarks  which  they  have  elicited  have  served  to  em- 
phasize to  me  the  too  evident  fact  of  the  general  scarcity 
of  practical  information  about  air-compression  and  the 
uses  of  compressed  air,  and  the  wide  diffusion  of  misinfor- 
mation  and  prejudice  upon  this  subject.  In  spite  of  it  all 
the  use  of  compressed  air  is  rapidly  spreading,  and  every- 
where  with  satisfaction  to  the  users.  I  would  gladly  do 
what  I  can  to  extend  the  field  of  its  usefulness,  and  I  have 
so  much  faith  in  its  powers  that  I  believe  that  the  best  of 
all  ways  to  advertise  it  is  simply  to  tell  the  straight  truth 
about  it,  and  that  I  have  tried  to  do. 

FRANK  RICHARDS. 

NEW  YORK,  May,  1895. 


CONTENTS. 


CHAP.  PAGE 

I.  MECHANICAL  versus  COMMERCIAL  ECONOMY i 

II.  DEFINITIONS  AND  GENERAL  INFORMATION , 9 

III.  A  TABLE  FOR  AIR-COMPRESSION  COMPUTATIONS 20 

IV.  THE  COMPRESSED-AIR  PROBLEM 27 

V.  THE  INDICATOR  ON  THE  AIR-COMPRESSOR 38 

VI.  THE  BEGINNING  OF  ECONOMICAL  AIR-COMPRESSION...  53 

VII.  OF  COMPRESSION  IN  A  SINGLE  CYLINDER 61 

VIII.  TWO-STAGE  AIR-COMPRESSION 70 

IX.  TWO-STAGE  COMPRESSION,    SINGLE-ACTING   TANDEM, 

ETC 8 1 

X.  THE  POWER  COST  OF  COMPRESSED  AIR 90 

XI.  THE  POWER  VALUE  OF  COMPRESSED  AIR 98 

XII.  COMPRESSED-AIR  TRANSMISSION no 

XIII.  THE  UP-TO-DATE  AIR-COMPRESSOR 126 

XIV.  COMPRESSED  AIR  versus  ELECTRICITY 135 

XV.  THE  THERMAL  RELATIONS  OF  AIR  AND  WATER 141 

XVI.  THE  FREEZING-UP  OF  COMPRESSED  AIR - 147 

XVII.  REHEATING  COMPRESSED  AIR 157 

XVIII.  COMPRESSED  AIR  FOR  PUMPING 167 

XIX.  THE  VARIOUS  APPLICATIONS  OF  COMPRESSED  AIR —  176 

v 


COMPRESSED  AIR. 


CHAPTER  I. 
MECHANICAL  VERSUS   COMMERCIAL  ECONOMY. 

BEFORE  considering  the  conditions  under  which  air  may 
be  most  economically  compressed,  having  regard  to  the 
power  cost  alone,  and  the  conditions  relating  to  the  trans- 
mission and  application  of  the  air,  so  that  the  most  power 
may  be  realized,  it  seems  proper  to  say  something  of  the 
many  applications  of  compressed  air  where  the  question  of 
the  actual  power  cost  of  the  air,  or  of  the  actual  amount 
of  power  realized  from  the  air,  seems  to  have  little  to  do 
with  the  case.  This  is  like  asking  a  suspension  of  judg- 
ment, or  a  reservation  of  final  decision  upon  the  claims  of 
compressed  air,  until  the  whole  case  has  been  presented  ; 
and  it  seems  to  be  rather  necessary,  because  so  many  have 
fallen  into  the  habit  of  thinking  only  of  the  losses  of  power 
in  the  use  of  compressed  air,  and  of  arguing  that  because 
certain  losses  are  proven,  that  therefore  the  employment  of 
compressed  air  is  not  to  be  considered  for  any  purpose. 
There  are  many  men  even  in  these  days,  and  many  intelli- 
gent engineers  among  them,  who,  to  their  own  loss,  will  not 


2  COMPRESSED    AIR. 

consider  the  claims  of  compressed  air  as  a  means  of  power 
transmission,  because  their  minds  have  been  so  filled  with 
this  idea  that  its  use  entails  enormous  losses.  That  con- 
sideration settles  it  for  them,  and  that  is  the  libel  from 
which  compressed  air  suffers,  so  that  it  does  not  get  a  fair 
chance  even  to  show  what  it  can  do.  It  is  only  another 
case  of  giving  a  dog  a  bad  name  ;  and  in  this  case  it  is 
a  very  good  dog  with  a  very  bad  name.  It  is  the  worst 
kind  of  a  case  to  set  right,  and  often  the  dog  dies  before 
justice  is  secured.  In  this  case,  however,  there  is  not  the 
slightest  possibility  of  killing  the  dog  or  of  shutting  him  out 
of  sight,  and  the  public  cannot  fail  in  the  end  to  get  hold  of 
the  facts  as  they  are. 

The  attitude  of  compressed  air  before  the  mechanical 
public,  and  especially  the  American  mechanical  public,  has 
been  a  peculiar  one  all  the  way  through.  It  has  had  no 
disinterested,  all-around  friends  to  look  after  its  interests, 
nor  interested  ones  either.  There  have  been  no  men,  and 
still  less  has  there  been  any  company  of  men,  who  have  made 
the  application  of  compressed  air  their  business  and  have 
looked  after  it.  Where  is  the  General  Compressed-Air  Com- 
pany, in  fact  as  well  as  in  name,  performing  for  compressed 
air  such  functions  as  more  than  one  company  are  performing 
for  electricity,  and  why  is  there  not  such  a  company?  Of 
the  builders  of  air-compressors  not  one  of  them  has  been, 
not  one  of  them  is  to  this  day,  responsible  for  the  econom- 
ical application  of  compressed  air  after  the  compression,  or 
apparently  cares  anything  about  it.  This  is  not  really  to  be 
wondered  at,  since  the  increase  in  the  use  of  compressed  air 
and  in  the  demand  for  compressors  has  been  so  great  and 
rapid,  at  least  in  the  United  States,  that  the  compressor 
builders  have  been  fully  occupied  in  satisfying  the  demand. 


MECHANICAL    VERSUS   COMMERCIAL  ECONOMY.      3 

Where  compressed  air  has  been  used  in  this  country,  and 
where  any  thought  has  been  given  to  economy  in  its  use, 
the  air-compressor,  it  would  seem,  has  been  almost  exclu- 
sively studied  and  talked  about.  In  the  progress  of  the 
steam-engine  it  has  sometimes  seemed  that  the  steam-boiler 
has  hardly  received  its  proper  share  of  attention.  In  the 
existing  writings  upon  steam-engine  economy  it  is  probably 
safe  to  say  that  the  engine  engrosses  ten  times  as  much  of 
the  matter  as  the  boiler  does.  In  the  case  of  compressed 
air  the  boiler,  or  compressor,  gets  ten  times  as  much  study 
and  discussion  as  the  engine  or  motor  or  other  apparatus 
which  uses  the  air.  There  are  such  vagaries  of  injustice  in 
civilized  communities. 

The  impression  which  has  got  abroad  of  the  waste  of 
power  in  the  use  of  compressed  air  has,  curiously  enough, 
been  fostered  and  disseminated  by  the  air-compressor  peo- 
ple more  than  by  any  one  else.  We  may  say  this  with  per- 
fect safety,  for  it  strikes  so  generally  that  it  hits  no  one  in 
particular.  The  compressed  air  literature  accessible  to  the 
general  public  consists  principally  of  air-compressor  cata- 
logues. The  argument  of  the  average  catalogue  runs  like 
this  :  "  If  you  are  going  to  use  compressed  air  for  any  pur- 
pose, look  out  for  the  enormous  losses  of  power  to  be  en- 
countered, and  which  you  are  sure  to  experience,  //  you 
don't  buy  our  compressor.'"  And  the  result  has  been  that 
many  have  "  caught  on  "  to  the  terrible  tale  of  the  waste  of 
power,  and  have  helped  to  spread  it  far  and  wide.  The 
argument  is,  of  course,  not  maliciously  meant ;  but  it  has 
done  more  work,  and  somewhat  different  work,  than  that 
which  was  intended. 

But  whether  the  employment  of  compressed  air  be  eco- 
nomical or  not,  as  far  as  the  application  of  the  power  is 


4  COMPRESSED    AIR. 

concerned,  we  do  not  propose  to  investigate  that  subject 
just  here.  We  wish  rather  to  call  attention  to  the  many 
applications  of  compressed  air  where  this  question  of  power 
economy  certainly  does  not  apply.  We  may  say,  indeed, 
that  in  a  large  majority  of  the  cases  in  which  compressed 
air  is  used  the  question  of  power  economy  does  not  apply. 
Even  in  the  use  of  compressed  air  for  driving  rock-drills  in 
mines  and  tunnels,  the  field  which  still  probably  employs 
one  half  of  all  the  compressed  air  that  is  used  for  mechani- 
cal purposes,  the  question  of  economy,  or  the  comparative 
cost  of  operation,  so  far  as  it  might  determine  the  employ- 
ment or  the  rejection  of  the  system,  is  not  worth  consider- 
ing, because  in  this  field  compressed  air  has  no  competitor, 
unless  hand-power  may  be  said  to  be  one.  In  the  use  of 
compressed  air  for  operating  the  brakes  upon  railway  trains, 
a  service  which  employs  a  greater  number  of  air-compres- 
sors than  any  other'  line  of  service  in  the  world,  economy 
of  power  is  not  to  be  considered.  Although  it  is  notorious 
that  the  compressors  employed  for  this  service  use  five 
times  the  steam  they  should  use  for  the  work  done,  and 
ten  times  as  much  steam  as  the  best-designed  air-com- 
pressors of  the  present  day  would  use  for  the  same  work, 
there  would  be  no  thought  of  throwing  the  air-brake  off  the 
trains  if  those  "  compressing-pumps,"  as  they  are  familiarly 
called,  used  double  or  four  times  as  much  steam  to  do  their 
work  as  they  use  now.  Any  one  of  several  collateral  con- 
siderations may  easily  take  precedence  of  or  assume  greater 
weight  and  importance  than  the  question  of  power  economy 
in  determining  the  employment  of  compressed  air.  The 
cases  are  few  in  which  it  is  employed  merely  as  a  means  of 
power  transmission,  and  in  competition  with  other  means 
of  power  transmission,  meeting  them  upon  equal  terms,  and 


MECHANICAL    VERSUS   COMMERCIAL   ECONOMY.      5 

with  no  other  consideration  but  that  of  the  comparative 
economy  with  which  the  power  is  transmitted.  There  are 
many  cases  that  are  clearly  cases  of  power  transmission,  or 
cases  of  work  to  be  done  at  a  distance  from  the  source  of 
power,  where  the  question  as  to  whether  the  power  is  to  be 
applied  continuously  or  intermittently  may  be  a  most  im- 
portant factor  in  the  general  problem.  Compressed  air  is 
unique  among  all  power  transmitters — at  least  among  long- 
distance power  transmitters — in  that  it  is  always  and  in- 
stantly ready  to  do  its  work  to  its  full  capacity,  and  yet 
that  it  charges  nothing  for  its  services  except  when  it  is 
actually  employed.  Other  transmitters  may  or  may  not 
propose  to  do  the  actual  work  a  little  cheaper,  but  they 
expect  to  be  an  expense  to  their  employer  for  maintenance 
when  not  employed.  In  the  pneumatic  switch  and  signal 
service  a  single  air-compressing  plant  is  capable  of  operating 
the  switches  and  signals  upon  a  section  of  railway  twenty 
miles  long  ;  and  when  the  pipes  are  once  filled  with  air  at 
the  required  pressure,  that  air  does  not  condense  or  suffer 
loss  or  deteriorate  in  any  way  except  as  it  is  used.  Elec- 
tricity makes  open  confession  of  its  inability  to  do  this 
intermittent  work,  in  that  while  in  this  switch  and  signal 
service  it  is  actually  employed  to  give  the  wink  to  the  air 
as  to  what  is  wanted,  the  air  has  to  do  the  work.  It  need 
not  be  said  that  steam  could  not  do  this  work,  for  its 
strength  would  all  be  turned  to  water  before  the  end  of  the 
pipe  was  reached. 

How  little  weight  the  actual  power  economy  may  have 
in  determining  the  employment  of  compressed  air  for  a 
given  purpose  is  suggested  by  the  conditions  of  the  pneu- 
matic postal  service.  In  the  cities  of  Europe  pneumatic 
postal  transmission  has  been  an  established  commercial  sue- 


6  COMPRESSED   AIR. 

cess  for  years,  and  recently  the  same  most  gratifying  experi- 
ence is  realized  at  the  Philadelphia  post-office.  The  appli- 
cation of  the  air  is  not  mechanically  economical,  only  ten 
per  cent  of  the  power  employed  being  applied  to  the  work 
of  moving  the  carriers,  while  ninety  per  cent  is  "  wasted  " 
in  accelerating  the  air  and  in  its  friction. 

The  "  compressing-pump  "  of  the  air-brake  service  being 
now  familiar  to  all  railroad  men,  and  being  at  hand  or  eas- 
ily procurable  by  all  railroad  shops,  has  been  of  late  years 
always  ready  in  those  shops  with  its  supply  of  compressed 
air,  and  this  ready  supply  of  compressed  air  has  led  to  the 
general  employment  of  it  in  railroad  shops  and  in  railroad 
service  for  a  variety  of  uses.  Whatever  it  is  tried  for,  its 
use  for  that  purpose  continues,  and  one  thing  leads  to  an- 
other. These  uses  of  compressed  air  in  railroad  shops  con- 
tinually increase,  not  because  the  air  is  more  adapted  to 
railroad  use  than  to  any  other,  but  because  the  air  is  there. 
Where  the  air  is  once  used  in  one  of  these  shops  its  use  in- 
creases, the  volume  of  air  used  increases,  and  the  increas- 
ing demand  for  the  air  is  met  by  setting  up  additional 
"  compressing-pumps "  in  succession,  until  in  some  in- 
stances as  many  as  eight  of  them  have  been  employed  to- 
gether to  supply  a  single  shop.  It  is  evident  here  that 
power  economy  could  have  little  to  do  with  the  case,  or 
some  thought  would  be  given  to  the  economical  compres- 
sion of  the  air,  and  a  good  air-compressor  would  take  the 
place  of  the  air-brake  pumps  as  a  means  of  supply.  This 
substitution  is  now  in  progress,  we  are  happy  to  say,  in 
many  shops  with  most  gratifying  results. 

A  recent  floating  paragraph  tells  how  the  master  me- 
chanic of  one  of  the  railroads  does  his  whitewashing  by 
compressed  air :  "  An  old  freight-car  has  been  fitted  with 


MECHANICAL    VERSUS   COMMERCIAL  ECONOMY.      7 

three  air-brake  pumps  and  two  reservoirs,"  instead  of  one 
air-compressor  and  one  reservoir  or  receiver,  "  the  pumps 
being  driven  by  steam  from  an  engine  (locomotive)  and  a 
pressure  of  40  pounds  being  maintained  in  the  reservoirs. 
The  car  and  engine  are  run  upon  the  track  alongside  the 
building  to  be  whitewashed.  By  a  system  of  piping  the  air 
is  carried  into  the  building,  and  to  the  long  iron  nozzles 
used  by  the  men  in  applying  the  fluid.  Each  man  has  an 
iron  tube  with  a  funnel-shaped  end,  from  which  the  white- 
wash is  sprayed  upon  the  woodwork.  To  each  nozzle  are 
attached  two  lines  of  hose,  one  supplying  the  air  and  the 
other  the  whitewash.  The  air  rushes  into  the  cylinder  of 
the  nozzle,  and  its  pressure  causes  a  suction  that  brings  up 
a  stream  of  whitewash  and  at  the  same  time  expels  it  in 
the  form  of  spray."  There  can  be  no  doubt  of  the  success 
of  this  scheme  ;  but  if  it  pays  to  use  a  locomotive,  a  freight- 
car,  three  brake-pumps,  two  reservoirs,  and  all  the  rest  of  it, 
for  such  a  job,  compressed  air  surely  must  be  cheap  at  any 
price,  or  there  must  be  economy  in  compressed  air  if  eco- 
nomically compressed  and  wisely  applied. 

As  to  how  much  the  question  of  power  economy  may 
have  to  do  with  the  remunerative  use  of  compressed  air  is 
suggested  by  an  article  in  Machinery,  September,  1894, 
describing  the  various  uses  to  which  compressed  air  is  put 
in  the  West  Shore  R.  R.  shop  at  Frankfort,  N.  Y.  "  It  is 
stated  that  the  entire  power  cost  for  running  the  air-com- 
pressors to  supply  the  whole  shop  is  not  more  than  ten 
cents  per  day,  while  the  actual  saving  in  labor  effected  is 
from  fifteen  to  twenty  dollars  per  day."  In  the  article  re- 
ferred to,  among  the  many  applications  of  compressed  air 
in  the  above  shop  mention  is  made  of  a  machine  for  (by 
the  aid  of  compressed  air)  putting  the  couplings  into  the 


8  COMPRESSED   AIR. 

ends  of  air-brake  hose.  "  This  little  machine  has  a  record 
of  putting  together  one  hundred  in  one  hour  and  five  min- 
utes, against  twenty-five  in  one  day  by  hand,  and  actually 
paid  for  its  entire  cost  in  one  day's  application." 

Simply  give  compressed  air  a  chance  and  it  will  quickly 
demonstrate  its  value.  The  progress  in  the  use  of  com- 
pressed air  thus  far  seems  plainly  to  indicate  that  its  wider 
application  is  promoted  more  by  having  a  supply  of  it  ready 
at  hand,  or  easy  to  get,  than  by  showing  how  cheaply  it  can 
be  furnished.  Those  who  are  introducing  small,  cheap 
compressors  that  work  with  reasonable  economy  are  doing 
excellent  missionary  work  for  compressed  air.  Farther 
along,  as  large,  permanent  compressing-plants  are  estab- 
lished, we  may  believe  that  the  minute  economies  will  re- 
ceive the  consideration  which  they  deserve.  However  the 
air  may  be  used,  and  however  profitable  it  may  be  to  use  it, 
it  will  always  be  in  order  to  get  it  as  cheaply  as  possible, 
and  economy  in  air-compression  must  always  be  a  clear 
gain. 


CHAPTER  II. 
DEFINITIONS  AND  GENERAL  INFORMATION. 

Ax  the  beginning  it  would  seem  to  be  well  to  get  to- 
gether for  use  or  reference  the  general  facts  in  relation  to 
air  and  to  the  phenomena  attending  its  compression.  As 
this  work  is  intended  for  the  greatest  good  of  the  greatest 
number,  being  for  popular  use,  or  for  the  use  of  those  who 
will  practically  have  to  do  with  compressed  air,  and  as  it  is 
in  no  sense  for  the  use  of  expert  scientists,  the  common 
standards  of  weight  and  measurement  will  be  employed 
wherever  it  is  possible  to  use  them  intelligibly.  For  tem- 
peratures the  Fahrenheit  scale  will  be  used  exclusively. 
All  measures  of  length  or  distance  will  be  given  in  feet 
and  inches,  and  weights  in  pounds  avoirdupois.  Where 
pressures  are  referred  to,  they  will  be  the  pressures  as  indi- 
cated upon  a  common  pressure  gauge,  or  the  pressures 
above  that  of  the  atmosphere.  The  absolute  pressure,  of 
course,  is  the  gauge  pressure  plus  the  pressure  of  the  at- 
mosphere at  the  given  time  and  place,  this  atmospheric 
pressure  being  usually  taken  as  14.7  Ibs.  at  the  sea-level. 
It  will  be  necessary  to  refer  to  absolute  pressures  occasion- 
ally, but  we  trust  that  no  misunderstanding  will  occur. 

Air  is  composed  of  23  parts  by  weight  of  oxygen  and  77 
parts  by  weight  of  nitrogen.  By  volume  the  proportions 
are  21  parts  of  oxygen  and  79  parts  of  nitrogen.  Although 

9 


IO  COMPRESSED    AIR. 

oxygen  is  thus  less  than  one  quarter  of  the  air,  it  is  much 
more  studied  and  written  about  and  is  apparently  of  much 
more  use  and  importance  than  the  larger  constituent.  It 
may  be  that  the  functions  of  nitrogen  are  not  yet  very  fully 
or  clearly  understood.  It  certainly  has  not  been  considered 
deserving  the  study,  nor  has  it  received  the  attention  that 
oxygen  has  received.  Oxygen  is  the  active  partner  in  the 
combination.  A  friend  who  is  blessed  with  more  knowl- 
edge in  this  line  than  is  possessed  by  the  writer  suggests 
that  oxygen  is  made,  for  the  mechanic  and  nitrogen  for  the 
farmer. 

We  shall  frequently  use  the  term  "  free  air  "  as  we  go 
along.  The  term  free  air  in  contradistinction  from  com- 
pressed air  is  only  used  as  a  matter  of  convenience  and 
custom.  Free  air,  or  air  at  atmospheric  pressure,  is  really 
compressed  air,  or  air  subjected  to  pressure,  as  truly  as  air 
at  100  Ibs.  pressure  is  compressed  air,  and  its  volume,  press- 
.ure,  and  temperature  are  subject  to  the  same  laws.  By  free 
air,  as  the  term  is  commonly  used,  is  meant  air  at  atmos- 
pheric pressure  and  at  ordinary  temperature,  and  it  is  the 
air  as  we  receive  it  when  we  begin  the  operation  of  air- 
compression.  It  is  free  air,  or  it  should  be  free  air,  when 
first  admitted  to  the  air-compressing  cylinder,  and  it  is  not 
free  air  again  until  it  is  exhausted  or  discharged  into  the 
atmosphere,  when  it  has  done  its  work  and  we  have  no 
further  use  for  it  or  connection  with  it. 

The  condition  in  which  our  free  air  is  received  is  not  by 
this  general  term  accurately  defined  in  any  of  its  particu- 
lars, either  as  to  pressure,  volume,  or  temperature.  The 
pressure  and  volume  of  the  air  may  vary  with  the  altitude 
or  location,  or  with  the  barometric  reading  in  any  given 
location,  or,  again,  the  volume  may  vary  with  the  tempera- 


DEFINITIONS  AND  GENERAL  INFORMATION.       II 

ture.  The  temperature  may  vary  with  the  changes  of  the 
seasons  or  with  the  special  surroundings.  For  general 
purposes  we  shall  assume  our  free  air  to  be  at  the  usual  sea- 
level  atmospheric  pressure  of  14.7  Ibs.,  absolute,  and  at  a 
temperature  of  60°. 

We  shall  have  to  refer  constantly  to  temperatures,  and, 
as  said  above,  the  Fahrenheit  scale  will  be  used  exclusively, 
and  usually  the  temperatures  will  be  the  sensible  tempera- 
tures, or  those  indicated  by  the  common  Fahrenheit  ther- 
mometer, 32°  being  the  melting-point  of  ice,  or  the  point 
where  water  changes  from  the  solid  to  the  liquid  state,  and 
212°  being  the  point  where  it  changes  from  the  liquid  to 
the  gaseous  state.  The  boiling-point  is  in  practice  quite  a 
variable  one,  and  depends  entirely  upon  the  pressure  sur- 
rounding the  water,  212°  being  the  boiling-point  only  at 
ordinary  atmospheric  pressures  near  the  sea-level.  Water 
may  theoretically  be  made  to  boil  at  any  temperature  above 
the  freezing-point  by  sufficiently  reducing  the  atmospheric 
pressure  to  which  it  is  exposed.  The  range  of  the  Fahren- 
heit scale  between  the  melting-point  of  ice  and  the  boiling- 
point  of  water  is  180  degrees. 

We  shall  have  frequent  occasion  to  refer  to  absolute  tem- 
peratures. Absolute  temperature  by  the  Fahrenheit  scale  is 
the  temperature  as  indicated  by  the  thermometer  plus  461 
degrees.  Thus  at  60°  by  the  thermometer  the  absolute 
temperature  is  60  -\-  461  =  521.  At  zero  the  absolute 
temperature  is  o  -\-  461  =  461.  At  temperatures  below  zero 
the  absolute  temperatures  are  also  determined  in  the 
same  way,  by  simple  addition.  Thus,  if  the  temperature 
by  the  thermometer  is  30°  below  zero,  or  —  30°,  the  ab- 
solute temperature  will  be  —  30  -j-  461  =  431.  In  all 
questions  relating  to  the  volume,  pressure,  or  weight  of 


12  COMPRESSED   AIR. 

air,  whether  compressed  or  not,  the  absolute  temperature 
of  the  air  has  an  important  bearing,  as  the  volume  of  the 
air  will  vary  directly  as  the  absolute  temperature,  and  the 
pressure  and  the  weight  of  the  air  will  all  have  changing 
relations.  If  the  absolute  temperature  of  the  air  is  in- 
creased, the  volume  will  be  increased  in  the  same  pro- 
portion, the  pressure  remaining  unchanged.  So  if  the 
absolute  temperature  of  the  air  be  diminished,  the  vol- 
ume will  be  diminished  in  the  same  way.  The  relations 
of  volume,  pressure,  and  temperature  of  air  are  thus  sum- 
marized : 

1.  The  absolute  pressure   of  air  varies  inversely  as  the 
volume  when  the  temperature  is  constant. 

2.  The  absolute  pressure  varies  directly  as  the  absolute 
temperature  when  the  volume  is  constant. 

3.  The  volume  varies  as  the  absolute  temperature  when 
the  pressure  is  constant. 

4.  The  product  of  the  absolute  pressure  and  the  volume 
is  proportional  to  the  absolute  temperature. 

A  cubic  foot  of  dry  air  at  atmospheric  pressure  and  at 
any  absolute  temperature  will  weigh  39.819  Ibs.  divided  by 
the  absolute  temperature.  Thus  at  60°  a  cubic  foot  of  air 
weighs  39.819  -i-  (60  +  461)  =  .0764  Ib.  So,  inversely,  the 
volume  of  i  Ib.  of  air  at  atmospheric  pressure  and  at  any 
absolute  temperature  may  be  ascertained  by  dividing  the 
temperature  by  39.819. 

Thus  at  60°  as  before  521  -f-  39.819  =  13.084  cu.  ft. 

The  following  table  (I),  from  Appleton's  Applied  Me- 
chanics, shows  the  weight  and  volume  of  air  at  different 
temperatures. 

If  the  temperature  and  the  pressure  of  air  both  vary  the 
constant  2.7093  multiplied  by  the  absolute  pressure  in  Ibs. 


DEFINITIONS  AND  GENERAL  INFORMATION.      1 3 


TABLE  I. 


TABLE   OF    THE   WEIGHT   AND   VOLUME   OF   DRY   AIR   AT  ATMOSPHERIC 
PRESSURE   AND   AT   VARIOUS   TEMPERATURES. 


From  Appleton's  Applied  Mechanics. 


Temperature, 
Degrees  Fahr. 

Weight  of 
One  Cubic  Foot 
in  Pounds. 

Volume  of 
One  Pound  in 
Cubic  Feet. 

0 

.0863 

11.582 

10 

.0845 

11.834 

20 

.0827 

12.085 

30 

.0811 

12.336 

32 

.0807 

12.386 

40 

.0794 

12.587 

50 

.0779 

12.838 

60 

.0764 

13-089 

70 

.0750 

13.340 

80 

.0736 

I3-592 

90 

.0722 

13.843 

100 

.0710 

14.094 

no 

.0697 

14-345 

1  20 

.0685 

14.596 

130 

.0674 

14.847 

140 

.0662 

15-098 

150 

.0651 

15.350 

160 

.0641 

15.601 

170 

.0631 

15.852 

1  80 

.0621 

16.103 

190 

.0612 

16.354 

2OO 

.0602 

16.605 

2IO 

•0593 

16.856 

212 

.0591 

16.907 

per  sq.  in.  and  divided   by  the  absolute  temperature  will 
give  the  weight  of  a  cubic  foot. 

What  will  be  the  weight  of  i  cu.  ft.  of  air  at  60  Ibs. 
pressure  and  100°  temperature? 

2.7093  X  (60  -f-  14.7)  -4-  (100  +  461)  =  .3607  Ib. 

The  volume  of  i  pound  of  air  may  be  obtained  by  di- 
viding the  absolute  temperature  by  the  absolute  pressure 
and  dividing  this  by  the  same  constant,  2.7093. 


H  COMPRESSED   AIR. 

What  will  be  the  volume  of  i  Ib.  of  air  at  75°  tempera- 
ture and  50  Ibs.  pressure  ? 

(75  +  461)  -f-  (50  +  14.7)  -T-  2.7093  =  3.052  cu  ft. 

If  the  temperature  of  the  air  is  changed  from  one  ab- 
solute temperature  T  to  another  absolute  temperature  t, 
the  volume  remaining  constant,  the  resulting  absolute 
pressure  p  may  be  obtained  from  the  original  absolute 
pressure  P  by  the  simple  proportion  T  \  t :  :  P  :  /,  or 
PX  t 

~Y~~p- 

It  is  not  supposed  that  heat  is  an  actual  existence  any 
more  than  sound  or  light  is ;  still  it  is  very  necessary, 
especially  in  all  matters  relating  to  compressed  air,  to  be 
able  to  accurately  measure  and  state  the  effects  of  heat, 
and  to  have  some  unit  or  standard  of  measurement  and 
comparison.  The  unit  of  heat  generally  adopted  is  that 
quantity  of  heat  that  will  raise  the  temperature  of  one 
pound  of  water  one  degree.  One  unit  of  heat  if  applied  to 
one  pound  of  anything  else  will  not  have  precisely  the  same 
effect  in  raising  the  temperature  that  it  has  when  applied 
to  water.  More  heat  is  required  to  raise  the  temperature 
of  one  pound  of  water  one  degree  than  is  required  for  any 
other  substance.  The  heating  effect  of  a  unit  of  heat 
applied  to  different  substances  is  found  to  vary  widely,  and 
the  special  quantity  of  heat  required  to  raise  the  tempera- 
ture of  one  pound  of  any  substance  one  degree  is  known  as 
its  specific  heat.  The  specific  heat  of  water  being  i,  the 
specific  heat  of  air  is  .2377,  or  the  same  unit  of  'heat  that 
would  raise  the  temperature  of  one  pound  of  water  one 
degree  would  raise  the  temperature  of  one  pound  of  air 
more  than  four  degrees.  The  application  of  heat  to  air  or 
to  any  elastic  fluid  may  have  either  of  two  effects.  It  may 


DEFINITIONS  AND  GENERAL   INFORMATION.      1 5 

increase  the  volume  while  the  pressure  remains  constant,  or 
it  may  increase  the  pressure  while  the  volume  remains  con- 
stant. The  specific  heat  will  be  quite  different  in  the  two 
cases.  The  specific  heat  of  air — -2377,  as  given  above — is 
its  specific  heat  at  constant  pressure,  and  the  heat  applied 
in  this  case  exhibits  its  effect  in  increasing  the  volume  of 
the  air.  If  the  air  be  confined  so  that  there  can  be  no  in- 
crease of  volume,  its  specific  heat  is  only  .1688,  or  about 
one  sixth  that  of  water.  Heat  applied  to  air  at  constant 
volume  increases  the  pressure  of  the  air.  If  heat  be  applied 
to  air  under  constant  pressure,  raising  its  temperature  from 
32°  to  212°,  the  increase  in  volume  will  be  from  i  to  1.367  ; 
and  if  heat  be  applied  to  air  at  constant  volume,  raising  its 
temperature,  as  before,  from  32°  to  212°,  the  increase  in 
absolute  pressure  will  be  from  i  to  1.365,  the  numerical 
result  being  practically  alike  in  the  two  cases,  but  the  heat 
expended  will  be  as  .2377  :  .1688,  or  nearly  one  half  more 
in  one  case  than  in  the  other. 

When  air  is  compressed,  or  when  its  volume  is  reduced 
by  the  application  of  force,  the  temperature  of  the  air  is 
raised.  This  phenomenon  occurs  entirely  regardless  of  the 
time  employed  in  the  compression.  If  during  the  compres- 
sion the  air  neither  loses  nor  gains  any  heat  by  communi- 
cation with  any  other  body,  the  heat  generated  by  the  act 
of  compression  remaining  in  the  air  and  increasing  its 
temperature,  then  the  air  is  said  to  be  compressed  adia- 
batically,  and  such  compression  is  adiabatic  compression. 
When  the  pressure  is  removed  from  the  air  and  it  is  allowed 
to  expand,  its  temperature  falls,  and  if  the  air  during  this 
operation  receives  no  heat  from  without,  it  is  said  to  expand 
adiabatically.  Adiabatic  compression  or  expansion  of  air 
is  compression  or  expansion  without  loss  or  gain  of  heat  by 


1 6  COMPRESSED   AIR. 

the  air.  This  expression  "  without  loss  or  gain  of  heat,"  it 
will  be  noticed,  does  not  mean  maintaining  a  constant  tem- 
perature, but  precisely  the  reverse  of  that. 

If  during  compression  the  air  could  be  kept  at  a  con- 
stant temperature  by  the  abstraction  of  the  heat  as  fast  as 
it  was  generated,  the  air  would  then  be  said  to  be  com- 
pressed isothermally.  In  isothermal  compression  or  ex- 
pansion the  air  remains  at  a  constant  temperature  through- 
out the  operation. 

The  rate  of  increase  in  the  temperature  of  air  during 
compression  is  never  uniform.  The  temperature  rises  faster 
during  the  earlier  stages  of  the  compression  than  when  the 
higher  pressures  are  reached.  Thus  in  compressing  from 
i  atmosphere  to  2  atmospheres  the  increase  of  temperature 
will  be  greater  than  in  compressing  from  2  to  3  atmospheres, 
and  so  on.  The  rate  of  increase  of  temperature  also  varies 
with  the  initial  temperature.  The  higher  the  initial  tem- 
perature the  greater  will  be  the  rate  of  increase  at  any  point 
and  throughout  the  compression. 

Attention  is  called  to  the  diagram  which  appears  as  a 
frontispiece  to  this  work.  It  is  taken  from  "  Compressed- 
Air  Production,"  by  W.  L.  Saunders,  C.E.  The  writer 
hereof  is  in  the  habit  of  keeping  this  diagram  in  sight,  nr<l 
finds  it  suggestive  and  handy  in  the  off  hand  solution  <  f 
many  questions  that  arise.  It  would  seem  to  be  worthy  •  ? 
a  rather  fuller  explanation  than  Mr.  Saunders  has  favoin.1 
us  with.  The  diagram  in  fact  comprises  two  distinct  dia- 
grams, the  one  showing  the  temperature  of  the  air,  and  the 
other  showing  the  volume  of  the  air  at  different  stages  of 
compression.  If  the  diagrams  are  understood,  there  is  no 
danger  of  confusing  the  one  with  the  other,  and  as  many  of 
the  lines  do  service  for  both  diagrams,  we  are  able  to  get 


DEFINITIONS  AND  GENERAL   INFORMATION.      I? 

much  from  a  small  space.  Compression  is  supposed  to 
commence  at  the  left  of  the  diagram  with  any  given  volume 
of  air  at  atmospheric  pressure.  As  the  compression  pro- 
ceeds the  successive  stages  of  pressure  are  indicated  by  the 
series  of  vertical  lines.  Beginning  at  the  extreme  left  witn 
the  gauge  pressure  at  zero,  o,  as  shown  at  the  bottom  of 
the  diagram,  or  with  an  absolute  pressure  of  i  atmosphere, 
as  indicated  at  the  top  of  the  diagram,  when  the  first 
vertical  line  is  reached  the  air  is  then  compressed  to  2 
atmospheres,  as  shown  by  the  figure  at  the  top,  or  to  14.7 
Ibs.  gauge  pressure,  as  shown  by  the  figures  at  the  bottom. 
When  the  next  vertical  line  is  reached,  the  air  has  been 
compressed  to  3  atmospheres,  absolute,  or  to  29.4  Ibs. 
gauge  pressure,  and  so  on,  the  diagram  extending  to  21 
atmospheres,  or  294  Ibs.  gauge  pressure  at  the  extreme 
right. 

In  connection  with  the  compression  of  air  the  important 
facts  to  be  known  are  the  temperature  of  the  air  when  any 
given  pressure  is  reached,  and  also  the  relative  volume  of 
the  air  when  compressed  to  any  given  pressure,  and  these 
points  it  is  the  function  of  the  diagram  to  show.  The 
curved  lines  running  from  the  lower  left-hand  corner  A  of 
the  diagram  are  the  lines  of  temperature,  and  they  indicate 
by  their  height  above  the  base-line  AB  the  temperature  of 
the  air  at  any  stage  of  the  compression.  It  is  assumed  in 
the  use  of  this  part  of  the  diagram  that  the  air  is  com- 
pressed adiabatically,  or  that  it  loses  none  of  the  heat  of 
compression  during  the  operation.  The  several  horizontal 
lines  of  the  diagram  serve  to  indicate  by  their  height  above 
the  base-line  AB  the  temperature  attained.  The  space 
between  any  two  adjacent  horizontal  lines  represents  100° 
of  temperature.  Thus  the  temperature  at  the  base-line 


1 8  COMPRESSED    AIR. 

AB  being  zero  the  temperature  at  the  first  line  above 
it  is  100°,  and  so  on.  The  temperatures  indicated  by  the 
lines  are  shown  by  the  figures  at  the  left  of  the  diagram 
along  the  vertical  starting-line  AC.  Intermediate  tempera- 
tures falling  between  the  lines  may  be  estimated  by  the 
relative  distances  from  the  lines.  The  temperature  of  the 
air  at  any  stage  of  the  compression  depends  upon  its  initial 
temperature.  The  higher  the  initial  temperature  is  the 
higher  will  be  the  temperature  throughout  the  compression. 
The  diagram  gives  temperature-lines  for  the  compression 
of  air  from  the  several  initial  temperatures  of  o°,  60°,  and 
100°.  These  lines  show,  as  noted  above,  that  the  higher 
the  initial  temperature  the  more  rapid  is  the  rise  through- 
out the  compression.  Thus  comparing  the  compression- 
line  from  o°  with  the  line  of  compression  from  100°  we 
notice  that  when  the  air  has  been  compressed  from  i 
atmosphere  to  10  atmospheres  the  original  difference  of 
100  degrees  has  become  200  degrees,  and  when  the  com- 
pression is  carried  to  20  atmospheres,  the  difference  has 
become  250  degrees. 

The  curved  lines  starting  downward  from  the  point  C  at 
the  upper  left-hand  corner  of  the  diagram  represent  the 
volume  of  any  unit  of  air  after  compression  to  any  given 
pressure.  The  upper  curved  line  represents  the  resulting 
volume  after  compression  without  any  cooling  of  the  air 
during  compression,  or  with  all  the  heat  of  compression 
remaining  in  the  air.  This  is  the  adiabatic  compression- 
line.  The  lower  or  inner  of  the  two  curved  lines  repre- 
sents the  volume  of  air  after  compression  to  any  given 
pressure,  and  with  all  the  heat  of  compression  abstracted 
immediately  as  it  is  developed,  or  with  the  air  constant  at 
the  initial  temperature  throughout  the  compression.  This 


DEFINITIONS  AND  GENERAL   INFORMATION.      19 

is  the  line  of  isothermal  compression.  The  initial  tem- 
perature of  the  air  whether  it  is  compressed  isothermally  or 
adiabatically  is  not  a  factor  in  determining  the  resulting 
volume.  The  total  height  of  the  starting-line  A  C  represent- 
ing i  volume  of  air,  the  volume  at  any  stage  of  the  com- 
pression is  represented  by  the  vertical  height  of  either 
curved  line  at  that  point.  The  several  horizontal  lines  ot 
the  diagram  serve  to  indicate  the  height,  and  by  the  height 
the  volume,  of  the  air  at  any  pressure,  and  in  this  relation 
the  lines  have  an  entirely  different  function  from  that 
borne  by  them  in  relation  to  the  lines  of  temperature. 
The  initial  volume  is  assumed  to  be  divided  into  ten  equal 
parts,  and  the  space  between  any  two  adjacent  horizontal 
lines  represents  one  tenth  of  the  original  volume.  Thus 
the  first  horizontal  line  below  the  top  line  CD  represents 
nine  tenths  of  the  initial  volume,  the  next  line  below  in- 
dicates eight  tenths  of  the  original  volume,  and  so  on. 
These  values  are  indicated  by  figures  at  the  right  of  the 
diagram  along  the  line  DB. 


CHAPTER  III. 
A  TABLE  FOR  AIR-COMPRESSION  COMPUTATIONS. 

THE  accompanying  Table  II,  which  the  writer  uses  con- 
stantly in  his  own  practice,  will  be  found  convenient  for 
working  up  indicator  diagrams  from  air-compressing  cylin- 
ders, and  in  general  computations  relating  to  air-compres- 
sion. It  should  require  little  explanation.  Throughout 
the  table  the  air  is  assumed  to  be  compressed  from  the 
normal  pressure  of  i  atmosphere, — 14.7  pounds, — and 
from  an  initial  temperature  of  60°  Fahrenheit.  The  first 
three  columns  of  the  table  are  of  course  different  forms  of 
the  same  thing — the  pressure  to  which  the  air  is  compressed. 
The  last  column  of  the  table  is  also  the  same  as  the  first 
merely  for  the  convenience  of  following  the  lines  of  figures. 
The  first  column  gives  the  pressures  as  they  would  actually 
be  shown  by  a  steam-  or  pressure-gauge.  It  would  be  the 
actual  available  working  pressure  of  the  air  after  compres- 
sion. The  second  column,  or  the  absolute  pressure,  is  ob- 
tained by  adding  the  normal  atmospheric  pressure — 14.7 
pounds — to  the  gauge  pressure.  The  third  column,  show- 
ing the  pressure  in  atmospheres,  is  obtained  by  dividing 
the  absolute  pressure  by  the  normal  atmospheric  pressure 
— 14.7  pounds. 

Column  4  gives  the  volume  of  air  (the  initial  volume 
being  i)  after  isothermal  compression  to  the  given  press- 


A   TABLE  FOR  AIR-COMPRESSION  COMPUTATIONS.    21 
TABLE   II. 

TABLE  OF  VOLUMES,  MEAN  PRESSURES,  TEMPERATURES,  ETC.,  IN  THE 
OPERATION  OF  AIR-COMPRESSION  FROM  I  ATMOSPHERE  AND 
60°  FAHR. 


g 

O 

!sL 

I 

&l 

11 

||i 

K 

3 

V 

i 

3 

8 

8 
< 

<g 
**• 

.h 
•<      ' 
.a 

3<L 

8-tS 

3-< 

*e 

JJ  c  c 

•g«^- 

§gf 

Si 

t 

3 

Gauge  Press 

Absolute  Pr 

Pressure  in 
pheres. 

Volume  wi 
Constant 
ture. 

Volume  wit 
Cooled. 

l<! 

fr,jS 

cl  | 

8s>H 

s 

Mean  Pre 
Stroke. 
Cooled. 

$  %  i  si 

0-&V3 

si-^S 

.U<a 

?13 
-§5 

jb 

a 

E 

Gauge  Press 

~ 

J 

i 

x 

~x 

0 

0 

0 

0 

60 

0 

! 

5-7 

i.  068 

•9363 

•95 

.96 

•  975 

•43 

•  44 

71 

2 

6.7 

1.136 

.8803 

.91 

1.87 

1.91 

•95 

•96 

80.4 

3 

7- 

1.204 

•  8305 

.876 

2.72 

2.8 

1.4 

1.41 

88.9 

4 

8. 

1.272 

.7861 

.84 

3-53 

3-67 

1-84 

1.86 

98 

5 

9- 

1.34 
1.68 

.7462 

.81 

7:02 

1:1 

2.26 

106 

15 

4- 
29- 

2.02 

•5952 
•495 

!oo6 

ii.5i 

5-77 

5-99 

178 

, 

20 

34- 

2.36 

•4237 

•543 

12.62 

14.4 

7-2 

7-58 

207 

20 

25 

39 

2-7 

•3703 

•494 

14-59 

17.01 

8-49 

9.05 

234 

25 

30 

35 
40 

44- 
49- 
54 

3-04 
3-721 

.3289 
•2957 
.  687 

.4638 
.42 
•393 

16.34 
17.92 
19.32 

19.4 

21.6 

23.66 

9.66 
10.72 
11.7 

10.39 
"•59 

12.8 

3? 

302 

35 
40 

45 

59- 

4.061 

.   462 

•37 

25-59 

12.62 

13  95 

45 

50 

64. 

4  401 

.  272 

•35 

21.79 

27-39 

J.3.48 

339 

50 

69. 
74 

4-741 

.   109 
.  968 

•331 
.3144 

22.77 
23-84 

29.11 
30.75 

14-3 
15  05 

15-98 
16.89 

357 

375 

65 

79- 

5.42; 

.  844 

.301 

24-77 

31.69 

,5-76 

17-88 

389 

65 

70 

84. 

5.762 

•   735 

.288 

26 

33-73 

16.43 

'8.74 

4°5 

70 

75 

89. 

6.IO2 

•   639 

.276 

26.65 

17.09 

19-54 

420 

80 
85 

94- 
99- 

J:* 

•   552 
•   474 

.267 
.2566 

27.33 
28.05 

37-94 

20  5 

31.22 

432 
447 

85 

9° 

104. 

7.122 

•   404 

.248 

28.78 

39.18 

18.87 

22 

459 

90 

95 

109. 

7.462 

•  34 

.24 

29-53 

40.4 

19.4 

32.77 

472 

95 

100 

14. 

7.802 

.  281 

.232 

30.07 

41.6 

19.92 

23-43 

485 

105 

19. 
24- 

8.142 

8.483 

.  228 
.   178 

:."3$ 

30.8. 
31-39 

42.78 
43-9' 

20.43 
20.9 

496 
507 

05 

10 

'5 

20 

29- 
34-7 

8.823 

9.163 

'33 
.1091 

.2129 
.2073 

3i.98 
32-54 

44.98 
46.04 

"i'.S94 

3:1* 

5i8 
529 

15 

25 

39-7 

9-503 

.1052 

.202 

33-07 

47.06 

22.26 

26.8! 

540 

25 

30 

44-7 

9-843 

•?% 

33-57- 

48.1 

22.69 

27  42 

550 

30 

35 

49-7 

o.  18^ 

.'0981 

34-05 

49-1 

23.08 

28.05 

560 

35 

40 

54-7 

0.523 

•095 

.1873 

34-57 

50.02 

23-41 

28.66 

570 

40 

45 

59-7 

o.f&i 

.0921 

-I837 

35-09 

5' 

23-97 

29.26 

580 

45 

50 

64-7 

1.204 

,0892 

.1796 

35-48 

51  .89 

24.28 

29.82 

589 

50 

60 

74-7 

1.88 

.084, 

.1722 

36.29 

53-65 

24-97 

30.91 

607 

60 

180 

94-7 

3  24 

.O79O 

•0755 

.1657 
•1595 

37-2 

37-96 

55-39 
57.01 

26.36 

32.03 
33-04 

O24 

640 

70 
So 

190 

204.7 

3-92 

.07.8 

.154 

38.68 

58.57 

27.02 

34-06 

657 

90 

200 

214.7 

4-6 

.0685 

.149 

39-42 

60.14 

27.7. 

35-02 

672 

200 

22  COMPRESSED   AIR. 

ure  ;  that  is,  assuming  that  the  temperature  of  the  air  has 
not  been  allowed  to  rise  during  the  compression,  or  that,  if 
the  air  has  not  been  completely  cooled  during  the  com- 
pression, it  has  been  cooled  to  the  initial  temperature 
after  the  compression.  In  this  case  the  volume  is  assumed 
to  be  inversely  as  the  absolute  pressure,  which  is  very 
nearly  correct.  The  figures  in  column  4  are  in  fact  re- 
ciprocals of  those  in  column  3,  and  they  are  obtained  by 
dividing  i  by  the  several  successive  values  in  column  3. 
Thus,  for  a  gauge  pressure  of  50  pounds,  the  volume  by 
isothermal  compression  should  be  i  -r-  4.401  =  .2272,  as 
given  in  column  4.  So  far  as  the  air-compressor  is  con- 
cerned, this  column  represents  an  eternally  unattainable 
ideal.  There  is,  and  as  far  as  we  can  see  there  can  be, 
no  absolutely  isothermal  compression.  Some  "  hydraulic  " 
compressors  are  claimed  to  accomplish  it,  but  while  there 
is  no  promise  of  their  practical  success  thy  have  no  right 
to  stand  in  evidence.  The  compressed  volume  while  in 
the  compressing  cylinder,  or  at  the  moment  of  discharge, 
will  always  be  greater  than  given  in  column  4  for  the  cor- 
responding pressure,  because  it  is  impossible  to  compress 
air  and  at  the  same  time  abstract  all  the  heat  of  com- 
pression from  it.  This  column  does,  however,  give  the 
volume  of  air  that  will  be  realized  if  the  air  is  trans- 
mitted to  some  distance  from  the  compressor,  or  if  it  is 
allowed  to  give  up  its  heat  in  any  way  before  it  is  used. 
Air  will  be  found  to  lose  its  heat  very  rapidly,  and  this 
column  may  be  taken  to  represent  the  volume  of  air  after 
compression  actually  available  for  the  purpose  for  which 
the  air  may  have  been  compressed. 

Column  5   of  the  table  gives  the  volume  of  air  at  the 
completion  of  the  compression,  assuming  that  the  air  has 


AIR-COMPRESSION  COMPUTATIONS.  2$ 

neither  lost  nor  gained  any  heat  during  the  compression, 
and  that  all  the  heat  developed  by  the  compression  remains 
in  the  air.  This  column  shows  the  air  more  nearly  as 
the  compressor  usually  has  to  deal  with  it,  although  the 
condition  represented  by  this  column — adiabatic  compres- 
sion— is  never  actually  realized,  any  more  than  isothermal 
compression  is  realized.  In  any  compressor  the  air  will 
lose  some  of  its  heat  during  the  compression,  and  the  air  is 
never  as  hot  during  the  compression,  nor  at  the  completion 
of  the  compression,  as  theory  says  that  it  should  be.  The 
theory  is  all  right,  but  the  air  loses  some  of  its  heat.  The 
slower  the  compressor  runs  the  better  chance  the  air  has  to 
give  up  some  of  its  heat,  and  consequently  the  smaller  will 
be  its  volume  all  through  the  operation,  and  the  less  will  be 
the  power  required.  High  or  excessive  speeds  are  not  in 
the  interest  of  economy  for  many  reasons.  If  the  cylinder 
and  the  cylinder-heads  are  water-jacketed,  the  cooling  of 
the  air  and  the  reduction  of  volume  and  of  mean  effective 
resistance  will  be  quite  appreciable  ;  but  in  general  prac- 
tice the  actual  volumes  of  air  at  the  completion  of  compres- 
sion will  be  found  to  be  nearer  the  figures  given  in  column 
5  than  to  those  in  column  4. 

Column  6  gives  the  mean  effective  resistance  to  be  over- 
come by  the  air-cylinder  piston  in  the  stroke  of  compres- 
sion, assuming  that  the  air  throughout  the  operation  re- 
mains constantly  at  its  initial  temperature — isothermal 
compression.  Of  course  the  air  never  will  remain  at  con- 
stant temperature  during  compression,  and  this  column 
remains  the  ideal  to  be  kept  in  view  and  striven  for  and 
continually  approximated  in  economical  compression. 

Column  7  gives  the  mean  effective  resistance  to  be  over- 


24  COMPRESSED   AIR. 

come  by  the  piston  for  the  compression  stroke,  supposing 
that  there  is  no  cooling  of  the  air  during  the  compression — 
adiabatic  compression.  As  we  have  seen,  there  is  more  or 
less — generally  less,  but  always  some — cooling  of  the  air 
during  its  compression,  so  that  the  actual  mean  effective 
resistance  will  always  be  somewhat  less  than  as  given  in 
this  column  ;  but  for  computing  the  actual  power  required 
tor  operating  air-compressor  cylinders  the  figures  in  this 
column  for  the  given  terminal  pressures  may  be  taken  and 
a  certain  percentage  added  for  friction, — say  5  per  cent, 
— and  the  result  will  represent  very  closely  the  power  re- 
quired by  the  compressor.  In  proposing  to  add  5  per  cent 
for  friction  we  do  not  mean  that  the  total  friction  of  a 
steam-actuated  air-compressor  will  be  only  5  per  cent,  for 
it  will  probably  be  more  than  10  per  cent,  but  part  of 
this  10  per  cent  will  have  been  compensated  for  by  the 
partial  cooling  of  the  air  during  the  compression.  In 
some  compressors  now  in  use  it  is  probable  that  so  much 
cooling  is  effected  during  the  compression,  and  so  much 
power  is  saved  thereby,  as  to  entirely  compensate  for  the 
friction  of  the  machine,  and  nothing  need  be  added  to  the 
result.  The  values  given  in  columns  6  and  7  are  of  course 
used  in  computing  the  horse-power  of  an  air-compressing 
cylinder  precisely  as  the  mean  effective  pressure  per 
stroke  in  a  steam-cylinder  is  used  in  computing  its  power. 
In  the  steam-cylinder  the  computation  gives  the  power 
developed  by  the  steam,  and  the  same  system  of  com- 
putation applied  to  the  air-cylinder  gives  the  power  used  in 
the  compression. 

Having  an  air-compressing  cylinder  20"  dia.  X  2'  stroke 
at  75   revolutions  per  minute,  or  300'  piston  speed,  com- 


AIR-COMPRESSION  COMPUTATIONS.  2$ 

pressing  air  adiabatically  to   75  Ibs.,  the  horse-power  used 
will  be  computed  as  follows  : 

20'  X  .7854  X  35.23  X  300  -f-  33,000  =  100  H.-P. 

It  may  be  proper  to  suggest  here  one  caution  as  to  the 
use  of  the  mean  effective  pressures  given  in  columns  6  and 
7.  The  pressures  given  being  for  compression  to  different 
pressures  from  an  initial  pressure  of  i  atmosphere,  it 
does  not  follow  that  those  values  will  be  correct  for  com- 
putations in  compound  compression,  or  for  compression 
from  any  other  initial  pressure  but  that  of  i  atmosphere. 
Thus  in  column  7  the  M.E.P.  for  compressing  from  i 
atmosphere  to  50  Ibs.  gauge  pressure  is  27.39.  ^n  ^is  case 
the  pressure  of  the  air  compressed  is  increased  50  Ibs.,  but 
it  does  not  follow  that  we  can  take  air  at  50  Ibs.  and  com- 
press it  to  100  Ibs.  with  the  same  mean  effective  pressure. 
In  the  latter  case  the  M.E.P.  required  would  be  40.33,  or 
47  per  cent  greater  than  in  the  former  case. 

Column  8  gives  the  mean  effective  resistance  for  the 
compression  part  only  of  the  stroke  in  compressing  air 
isothermally  from  a  pressure  of  i  atmosphere  to  any 
given  pressure.  This  at  once  calls  our  attention  to  the  two 
distinct  operations  involved  in  practical  air-compression  : 
the  actual  compression  of  the  air  to  the  given  pressure,  and 
the  delivery  or  expulsion  of  the  air  from  the  cylinder  after 
the  full  pressure  is  attained.  These  two  operations  corre- 
spond inversely  to  the  two  operations  occurring  in  the 
cylinder  of  a  steam-engine  :  the  admission  of  the  steam, 
where  it  is  sustained  at  approximately  full  pressure  until 
the  point  of  cut  off,  and  the  expansion  of  the  steam  from 
the  point  of  cut  off  to  the  termination  of  the  stroke,  the 
expansion  period  in  the  steam-cylinder  of  course  corre- 


26  COMPRESSED   AIR. 

spending  inversely  with  the  compression  in  the  air-cylinder, 
and  the  admission  of  the  steam  corresponding  with  the 
delivery  of  the  air. 

It  will  be  noticed  that  the  mean  effective  pressures  in 
columns  8  and  9,  for  the  compression  part  only  of  the 
stroke,  are  much  lower  than  those  in  columns  6  and  7  for 
the  whole  stroke,  but  when  to  the  work  of  the  compression 
part  of  the  stroke  is  added  the  work  of  delivery,  the  values 
will  be  found  to  correspond  very  nearly.  Thus  when  com- 
pressing adiabatically  to  50  pounds  gauge  pressure  the 
volume  of  air  delivered  will  be  (column  5)  .35  of  the  origi- 
nal volume,  or  .35  of  the  stroke  for  each  cylinderful  of 
free  air,  so  that  the  pressure  or  resistance  for  .35  of  the 
stroke  will  be  50  Ibs.,  while  for  the  compression  part  of  the 
stroke — i  —  .35  =  .65 — the  resistance  will  be  15.05,  as  given 
in  column  9.  Then  (15.05  X  .65)  +  (50  X  .35)  =  27.28, 
which  corresponds  as  well  as  could  be  expected  with  the 
value  in  column  7  for  the  whole  stroke — 27.39. 

There  is  also  to  be  observed  a  less  proportional  differ- 
ence between  the  values  in  columns  8  and  9  than  between 
those  in  columns  6  and  7,  but  this  also  will  be  found  to  be 
compensated  for  by  the  differences  in  terminal  volume  for 
isothermal  or  for  adiabatic  compression  and  the  different 
proportion  of  the  stroke  occupied  by  the  full  pressure  of 
delivery.  Thus  comparing  the  figures  for  isothermal  com- 
pression with  those  just  given  for  adiabatic  compression, 
compressing  to  50  Ibs.  as  before,  we  have  :  (13.48  X  .7728) 
-f-  (50  X  .2272)  =  21.78,  a  result  which  may  be  said  to  be 
identical  with  the  value  21.79  for  the  whole  stroke,  as  given 
in  column  6. 

Columns  8  and  9,  as  will  be  referred  to  later,  will  be 
found  serviceable  in  computing  the  power  used  in  the  first 


AIR-COMPRESSION  COMPUTATIONS.  2"J 

stage  of  compound  compression,  where  generally  the  entire 
function  of  the  first  cylinder  is  that  of  compression  only, 
its  total  contents  from  the  beginning  to  the  end  of  the 
stroke  being  simply  compressed  into  the  volume  contained 
in  the  smaller  cylinder,  and  there  being  no  part  of  the 
stroke  properly  occupied  in  delivery  or  expulsion  at  any 
completed  pressure. 

Column  10  gives  the  theoretical  temperature  of  the  air 
after  compression,  adiabatic,  to  the  given  pressure.  As  we 
have  remarked  elsewhere,  the  actually  observed  tempera- 
ture in  these  cases  is  never  as  high  as  the  theoretical  tem- 
perature. This  is  not  that  the  theory  is  incorrect,  for,  as 
usual,  the  theory  is  more  nearly  correct  than  "  practical " 
people  are  wont  to  allow.  If  the  temperature  of  the  com- 
pressed air  by  observation  is  not  found  to  correspond  with 
the  figures  as  given,  it  is  only  because  the  air  is  being  cooled 
by  conduction  or  radiation  even  while  it  is  being  heated  by 
compression. 


CHAPTER  IV. 
THE  COMPRESSED-AIR  PROBLEM. 

THE  general  problem  of  air-compression  and  of  the  ap- 
plication of  compressed  air  to  the  re-development  of  power 
may  be  stated  in  simple  terms.  Fig.  Fta.l 

i  really  tells  the  whole  story.  The 
piston  F  is  fitted  to  the  cylinder  £, 
so  that  we  may  assume  it  to  move 
freely  and  without  leakage.  The 
piston  being  at  A,  as  shown,  and 
the  cylinder  being  filled  with  air 
at  a  pressure  of  i  atmosphere,  and 
at  normal  temperature,  a  sufficient 
weight  is  placed  upon  the  piston  to 
force  it  down  into  the  cylinder  and 
compress  the  air  contained  in  it  to 
a  pressure  of,  say,  6  atmospheres. 
The  volume  being  inversely  as  the 
pressure,  the  piston  should  go  down 
to  C.  We  find,  however,  that  it  ac- 
tually only  goes  down  to  £,  and  the 


reason  is  that  while  the  air  is  being  compressed  the  opera- 
tion of  compression  also  heats  it,  and  the  heat  distends  or 
expands  the  air,  and  its  volume  is  consequently  consider- 
ably greater  than  it  should  be,  upon  the  assumption  that 
the  volume  is  always  inversely  as  the  pressure. 

28 


THE    COMPRESSED-AIR  PROBLEM.  29 

This  is  an  illustration  of  the  frequent  differences  that 
arise  between  theory  and  practice,  with  the  usual  result 
that  practice  is  all  right,  and  that  theory  will  be  in  perfect 
accord  with  it  when  the  theory  in  the  case  is  complete. 
Theory  thus  far  had  not  thought  of  the  temperature  of  the 
air. 

Supposing  both  the  piston  and  the  cylinder  to  be  abso- 
lute non-conductors  of  heat,  and  that  the  air  heated  by  the 
compression  loses  none  of  its  heat  of  compression,  then  if 
the  weight  which  forced  the  piston  down  to  B  be  taken 
away,  the  piston  will  be  driven  back  to  its  original  position 
at  A,  and  the  air  contained  in  the  cylinder  will  have  re- 
sumed its  normal  pressure  and  temperature,  and  will  have 
done  as  much  work,  or  will  have  exerted  as  much  force,  by 
its  return,  as  was  employed  in  the  act  of  compression.  If 
while  the  piston  was  at  B,  and  with  the  weight  upon  it  suf- 
ficient to  balance  the  pressure  of  6  atmospheres,  the  air 
by  any  means  had  been  cooled  to  its  original  temperature, 
the  piston  would  have  fallen  to  C,  and  the  law  that  the  vol- 
ume varies  inversely  as  the  pressure  would  have  held  good, 
for  then  the  initial  and  the  final  temperatures  would  have 
been  the  same.  The  air  being  thus  cooled  to  its  original 
temperature,  and  the  piston  being  at  C,  upon  removing  the 
weight  from  the  piston  it  would  return  only  to  D,  instead 
of  to  A.  When  the  piston  arrived  at  D,  the  pressure  of 
the  air  in  the  cylinder  would  have  fallen  to  the  original 
pressure  of  i  atmosphere,  and  the  piston  at  D  would  be 
balanced  between  the  pressures  above  and  below  it.  As 
the  air  is  heated  in  the  operation  of  compression,  so  is  it 
correspondingly  cooled  during  its  expansion,  and  when  the 
piston  reaches  D  the  air  in  the  cylinder  is  then  at  atmospheric 
pressure,  because  it  is  then  much  colder  than  it  was  at  the 


3O  COMPRESSED   AIR 

beginning ;  and  it  is  solely  because  of  this  loss  of  heat  that 
the  pressure  falls  so  early,  and  that  the  piston  does  not  re- 
turn to  A  where  it  started  from.  If  while  the  piston  is  at 
D  the  air  can  by  any  means  recover  all  the  heat  which  it 
has  lost,  the  piston  will  return  to  A  as  before.  The  dis- 
tance DA  compared  with  CA,  or  the  distance  DC,  repre- 
sents the  total  possible  theoretical  loss  of  power  in  the  com- 
pression and  the  re-expansion  of  air. 

At  the  risk  of  anticipating  a  number  of  points  that  I 
hope  to  bring  out  more  fully  and  in  detail  later  on,  we  may 
now  refer  to  the  more  or  less  practical  diagram  Fig.  2. 
This  diagram,  scale  40,  is  intended  to  show  the  practical 
possibilities  in  the  use  of  compressed  air  at  75  Ibs.  gauge,  or 
6  atmospheres.  The  line  ab  is  the  adiabatic  compression- 
line,  or  the  line  of  compression,  upon  the  assumption  that 
no  heat  is  taken  away  from  or  is  lost  by  the  air  during  the 
compression.  The  initial  temperature  of  the  air  being  60 
degrees,  the  final  temperature  would  be  about  415  degrees, 
and  the  final  volume  would  be  .28  of  the  original  volume. 
The  line  ac  is  the  isothermal  compression-line,  which  as- 
sumes that  all  the  heat  of  compression  is  got  rid  of  just 
when  it  is  produced,  or  that  the  air  throughout  the  com- 
pression remains  constantly  at  its  initial  temperature.  The 
final  volume  in  this  case  is  .1666  of  the  original  volume. 
Remembering  that  these  lines,  ab  and  ac,  represent  the 
compression  of  the  same  initial  volume  of  air,  it  is  evident 
that  there  is  quite  a  difference  in  the  amount  of  power  em- 
ployed in  the  two  cases,  and  herein  lies  the  loss,  or  the  pos- 
sibility of  loss,  of  power  in  the  operation  of  compression. 
The  mean  effective  pressure  or  resistance  of  the  air  for  the 
stroke  upon  the  adiabatic  line  abl  is  35.36  Ibs.,  while  the 
mean  effective  pressure  for  the  isothermal  compression-line 


THE   COMPRESSED-AIR   PROBLEM. 


is  but  27  Ibs.,  or  only  76  per  cent  of  the  former.     The 
comparison  should,  however,  be  reversed.     The  adiabatic 


mean  effective  pressure  is  131  per  cent  of  the  isothermal 
mean  effective  pressure  : 

27  :  35.36  ::  i  :  1.31, 

and  this  31  per  cent  is,  of  course,  the  additional,  or,  as  we 
might  say,  the  unnecessary,  power  employed,  assuming  iso- 


32  COMPRESSED   AIR. 

thermal  compression  to  be  attainable.  Neither  of  these 
compression-lines,  ab  or  at,  is  possible  in  practice.  Air 
cannot  be  compressed  without  losing  some  of  its  heat  dur- 
ing compression,  so  that  the  actual  compression-line  must 
always  fall  within  or  below  the  line  ab.  On  the  other 
hand,  it  is  equally  impossible  to  abstract  all  the  heat  from 
the  air  coincidently  with  the  appearance  of  that  heat,  so 
that  the  actual  compression-line  must  always  fall  outside 
or  above  the  line  ac.  The  best  air-compressor  practice  of 
to-day  is  very  near  the  line  ao,  or  the  mean  of  the  adiabatic 
and  the  isothermal  curves.  The  actual  line  is  generally 
above  this,  and  seldom  below  it.  It  would  be  impossible  to 
produce  a  line  exactly  coincident  with  this  in  practice.  If 
we  produced  a  line  giving  the  same  mean  effective  pressure 
as  ao,  it  would  probably  run  above  ao  for  the  first  half  of  the 
stroke,  and  perhaps  a  little  below  it  at  the  last.  If  the  com- 
pression were  two-stage  or  compound, — that  is,  if  it  were 
done  in  two  or  more  cylinders  instead  of  in  one, — there 
would  of  course  be  breaks  in  the  continuity  of  the  com- 
pression-curve. The  mean  effective  pressure  for  the  line 
aol  is  about  31.5  Ibs.,  or  still  nearly  17  per  cent  in  excess  of 
the  M.E.P.  for  the  line  acl.  As  aol  represents  exceptionally 
good  practice,  the  loss  of  power  for  the  average  practice  in 
air-compression,  independently  of  the  friction  of  the  ma- 
chinery, may  be  put  at  20  per  cent.  Lest  some  impatient 
ones  should  drop  the  subject  here,  or  lest  some  rival  of 
compressed  air  should  pick  up  this  and  run  away  with  it, 
we  might  insert  a  reminder  here  that  all  this  loss  is  not 
necessarily  final.  In  all  these  comparisons  for  efficiency 
the  actual  compression-line  is  always  to  be  compared  with 
the  isothermal  line  ac/,  because  that  is  the  ideal  line  for 
compression  without  loss  of  power,  and  because  the  termi- 


THE   COMPRESSED-AIR  PROBLEM,  33 

nal  volume  cl  is  the  volume  actually  available  for  use,  no 
matter  how  economically  or  wastefully  the  air  may  have 
been  compressed.  Though  at  the  completion  of  the  com- 
pression stroke  there  is  always  some  of  the  heat  of  com- 
pression remaining  in  the  air,  and  its  volume  is  always 
greater  than  cl,  that  heat  is  always  lost  in  the  transmission 
of  the  air,  or  in  its  storage,  and  the  available  volume  is 
never  practically  above  cl. 

After  the  cooling  and  contraction  of  the  compressed  air 
comes  the  question  of  loss  in  the  transmission  of  it.  To 
cause  the  air  to  flow  through  the  pipe  there  must  be  some 
excess  of  pressure  at  the  first  end  of  it,  a  constant  decrease 
of  pressure  as  the  air  advances,  and  consequently  a  loss  of 
available  power  at  the  delivery  end.  But  this  loss  has  been 
greatly  exaggerated.  Here,  as  in  other  matters,  the  air-com- 
pressor builders  have — unwittingly,  we  will  say — done  more 
harm  than  good  as  regards  the  interests  of  compressed  air. 
Formidable  tables  are  in  all  the  air-compressor  catalogues, 
showing  the  loss  of  pressure  due  to  the  friction  of  air  in 
pipes.  The  tables  are  not  dangerous,  and  are  not  published 
primarily  for  the  purpose  of  frightening  timid  investors. 
They  are  only  intended  to  suggest  the  size  of  pipe  most 
suitable  for  any  given  case  of  transmission.  If  they  tell  us 
truly  of  the  loss  of  pressure,  they  still  fail  to  tell  us  that  the 
loss  of  pressure  is  not  necessarily,  or  to  the  same  extent,  a 
loss  of  power.  The  actual  truth  is  that  there  is  very  little 
loss  of  power  through  the  transmission  of  compressed  air  in 
suitable  pipes  to  a  reasonable  distance,  and  the  reasonable 
distance  is  not  a  short  one.  With  pipes  of  proper  size,  and 
in  good  condition,  air  may  be  transmitted,  say,  ten  miles, 
with  a  loss  of  pressure  of  less  than  i  Ib.  per  mile.  If  the 
air  were  at  80  Ibs.  gauge,  or  95  Ibs.  absolute,  upon  entering 


34  COMPRESSED    AIR. 

the  pipe,  and  70  Ibs.  gauge,  or  85  Ibs.  absolute,  at  the  other 
end,  there  would  be  a  loss  of  a  little  more  than  10  per  cent 
in  absolute  pressure,  but  at  the  same  time  there  would  be 
an  increase  of  volume  of  n  per  cent  to  compensate  for  the 
loss  of  pressure,  and  the  loss  of  available  power  would  be 
less  than  3  per  cent.  With  higher  pressures  still  more  fa- 
vorable results  could  be  shown. 

Having  compressed  the  air  and  conveyed  it  to  the  point 
where  we  wish  to  use  it,  we  may  turn  again  to  Fig.  2,  and 
see  what  we  will  be  able  to  do  with  the  air.  The  air  may 
be  used  in  various  ways  with  widely  different  economic 
results,  and  little  ingenuity  is  required  to  accomplish  enor- 
mous losses.  Having  the  volume  cl,  and  using  it  in  a  cyl- 
inder of  suitable  capacity,  cutting  off  so  as  to  expand  down 
to  i  atmosphere  before  release,  the  adiabatic  expansion- 
line,  or  the  lowest  line*  that  the  air  could  make,  would  be 
the  line  cd,  and  the  total  loss  in  the  use  of  the  air,  as  com- 
pared with  the  power  cost  of  compressing  it,  would  be  the 
difference  between  the  areas  aolh  and  Icdh^  the  latter  being 
66  per  cent  of  the  former. 

The  temperature  of  the  air  at  c,  where  the  expansion  be- 
gins, being  assumed  to  be  60°,  the  cooling  of  the  air  which 
always  accompanies  its  expansion  will  bring  the  tempera- 
ture far  down  the  scale  when  d  is  reached,  d  being,  of 
course,  the  end  of  the  cylinder  wherein  the  expansion  takes 
place.  The  theoretical  temperature  of  the  air  at  the  end 
of  the  stroke  would  be  about  —  150°.  The  actual  tempera- 
ture in  these  cases  is  never  found  as  low  as  the  theoretical 
temperature,  because  the  air  receives  heat  from  the  cylin- 
der and  from  the  walls  of  the  passages  with  which  it  comes 
in  contact ;  but  it  is  usually  still  cold  enough  to  cause  seri- 
ous inconvenience  in  practice,  and  this  cooling  of  the  air 


THE   COMPRESSED-AIR   PROBLEM.  35 

may  in  many  cases  be  fatal  to  its  employment,  entirely  re- 
gardless of  the  economy  of  the  case.  The  air  almost  inva- 
riably contains  moisture,  the  amount  varying  with  the  sur- 
rounding meteorological  conditions,  and  as  the  air  becomes 
attenuated  and  so  intensely  cold  the  water  is  rapidly  frozen 
in  the  passages,  and  soon  chokes  them  up  and  stops  the 
operation  of  the  motor.  The  prevention  or  the  circumven- 
tion of  the  freezing  up  of  air  apparatus  is  an  additional 
complication  of  the  compressed-air  problem  to  be  con- 
sidered later. 

The  trouble  from  the  freezing  up  naturally  suggests  the 
heating  of  the  air  before  it  is  used.  The  heating  or  re- 
heating of  the  air,  where  it  is  practised,  not  only  brings  us 
out  of  our  trouble  about  the  freezing  up,  but  it  increases 
the  volume  of  the  air  and  its  consequent  available  power  at 
a  very  slight  expense  for  the  heating.  If  the  volume  of 
air  cl,  being  now  at  60°,  be  passed  through  a  suitable  heater 
and  its  temperature  raised  to  300°,  its  volume  will  then  be 
il,  instead  of  c/,  or  .2434  instead  of  .1666,  an  increase  of 
volume  of  about  50  per  cent.  In  practice,  to  insure  a  tem- 
perature of  300°  in  the  cylinder  at  the  beginning  of  the  ex- 
pansion, it  will  be  necessary  to  heat  the  air  considerably 
above  that  temperature,  say  to  400°,  as  the  air  loses  its  heat 
very  rapidly.  If  now  we  use  this  reheated  air,  the  volume 
cl  then  becoming  il,  and  expanding  this  air  down  to  e,  sup- 
posing the  temperature  at  /to  be  300°,  the  theoretical  fina, 
temperature  will  be  about  zero.  The  actual  temperature, 
it  is  pretty  certain,  will  not  be  below  the  freezing-point, 
and  all  our  trouble  about  the  freezing  of  the  passages  will 
have  disappeared,  and  the  power  realized  will  have  been 
much  increased.  It  seems  to  be  quite  practicable,  by  ef- 
fective cooling  of  the  air  during  its  compression,  and  by 


36  COMPRESSED   AIR. 

reheating  it  before  its  re-expansion,  to  bring  the  expansion- 
line  ie  to  enclose  an  area  not  less  than  that  enclosed  by  the 
compression-line  ao,  and  then  the  entire  losses  will  be  those 
attributable  to  the  clearances  and  to  friction.  It  is  said 
that  in  practice  85  per  cent  of  the  initial  power  has  already 
been  realized  after  transmitting  the  air  to  considerable  dis- 
tances. "  It  is  said  "  accomplishes  many  wonderful  feats. 

It  was  remarked  above  that  the  air  after  compression 
and  transmission  might  be  employed  with  widely  different 
economic  results.  As  an  instance  of  "how  not  to  do  it  "  I 
might  cite  the  case,  of  too  frequent  occurrence,  where  air 
is  delivered  to  a  mine  for  operating  rock  drills  and  other 
mining  machinery,  and  air  then  taken  from  the  same  pipe- 
line for  operating  a  pump.  This  practice  would  be  all 
right  if  the  pump  were  adapted  to  the  work  to  be  done  and 
to  the  pressure  of  air  carried.  The  pump,  however,  is  gen- 
erally a  common  direct-acting  steam-pump,  with  all  that 
the  term  implies,  and  which  has  been  obtained  without  any 
reference  to  the  economical  use  of  the  air.  As  it  has  prob- 
ably already  been  run  by  steam,  or  is  designed  to  be  run 
by  steam,  it  calls  for  a  low  operating  pressure  ;  this  being 
a  necessity  on  account  of  the  condensation  and  loss  of 
pressure  in  steam  when  transmitted  through  long  pipes. 
Say  that  the  compressed-air  pipe  carries  75  Ibs.  pressure, 
while  the  pump  only  requires  25  Ibs.  It  would  be  an  ad- 
vantage in  a  case  like  this  to  use  a  pressure-reducer  in  the 
supply-pipe  at  a  considerable  distance  from  the  pump,  so 
that  the  expansion  to  the  lower  pressure  required  might 
take  place,  and  the  air  have  an  opportunity  to  recover  its 
temperature  and  volume  before  going  into  the  pump- 
This,  however,  is  seldom  attended  to,  and  the  required 
reduction  of  pressure  is  effected  entirely  by  the  throttle- 
valve  at  the  instant  of  admission.  The  available  power, 


THE    COMPRESSED-AIR  PROBLEM.  37 

then,  when  the  air  is  so  employed,  will  be  represented  by 
the  area  pmnh  as  compared  with  the  area  ablh,  or,  at 
the  best,  aolh,  representing  the  power  that  was  expended 
in  compressing  the  air.  Then,  if  we  deduct  the  losses 
attributable  to  the  useless  filling  of  the  large  clearances  of 
the  common  steam-pump,  and  to  the  leakages  that  are  the 
usual  accompaniment  of  such  generally  extravagant  prac- 
tices, it  is  little  wonder  that  compressed  air  is  held  in  low 
esteem.  Under  circumstances  far  from  the  most  unfavor- 
able I  have  found  pumps  realizing  only  15  per  cent  of  the 
power  expended  at  the  compressor,  and  I  have  no  doubt 
that  there  are  many  pumps  being  operated,  or  whose  oper- 
ation is  attempted,  where  not  more  than  10  per  cent  of  the 
original  power  is  realized  ;  and,  even  then,  when  the  use  of 
compressed  air  for  operating  such  pumps  under  such  con- 
ditions is  condemned,  it  is  apt  to  be  because  they  freeze  up 
and  won't  go,  rather  than  on  account  of  their  enormous 
waste  of  power.  From  the  fact  that  a  mining  pump  has  a 
lift  that  is  nearly  constant,  the  pump,  if  properly  propor- 
tioned and  adapted  to  its  work,  should  be  an  efficient  mis- 
sionary for  compressed  air,  rather  than  its  most  malignant 
traducer. 

The  word  "  loss  "  that  we  find  ourselves  using  in  connec- 
tion with  this  general  subject  should  not  be  allowed  to 
mislead  us.  The  use  of  compressed  air  is  for  the  accom- 
plishment of  a  desirable  purpose,  and  it  is  not  to  be  ex- 
pected that  such  a  purpose  can  be  effected  for  nothing. 
The  transmission  of  power  is  as  much  to  be  paid  for  as  the 
generation  of  the  power.  Where  water  power  is  used,  the 
means  of  transmission  may  be  the  principal  item  of  cost. 
Where  the  difference  between  the  power  expended  and  the 
power  realized  is  not  excessive,  that  difference  is  simply  a 
fair  price  paid  for  a  good  service  rendered,  and  there  is  no 


COMPRESSED    AIR. 


loss  about  it.  Where  losses  do  occur  in  the  use  of  com- 
pressed air,  they  are  like  the  losses  which  occur  in  business, 
and  which  cut  short  many  a  brilliant  career.  Power  is  lost 
simply  because  it  is  not  saved,  and  the  means  of  saving  are 
not  hard  to  find  nor  far  to  seek.  The  excessive  losses  are 
not  necessary  nor  unavoidable,  nor  without  compensation. 
A  failure  to  understand  and  appreciate  this  situation  im- 
pedes the  progress  of  compressed  air. 
TABLE  III. 

TABLE  OF  FINAL  TEMPERATURES  OF  AIR  COMPRESSED  ADIABATIC/.LLY 
TO  DIFFERENT  GAUGE  PRESSURES  FROM  AN  INITIAL  PRESSURE  OF 
I  ATMOSPHERE,  AND  FROM  DIFFERENT  INITIAL  TEMPERATURES. 


Final  Pressure 
Gauge. 

Initial  Temp. 

0°. 

Initial  Temp. 
32°. 

Initial  Temp. 
60°. 

Initial  Temp. 

I 

8 

41 

70 

Ill 

2 

16 

50 

79 

121 

3 

25 

59 

88 

132 

4 

33 

67 

97 

140 

5 

4i 

75 

106 

150 

10 

74 

"3 

144 

191 

15 

104 

144 

177 

226 

20 

130 

171 

207 

258 

25 

153 

196 

233 

287 

30 

175 

219 

258 

313 

35 

195 

240 

280 

337 

40 

213 

260 

301 

360 

45 

231 

279 

321 

38i 

50 

247 

296 

339 

401 

55 

262 

316 

357 

420 

60 

277 

328 

373 

437 

65 

291 

343 

389 

454 

70 

304 

358 

404 

471 

75. 

317 

371 

419 

486 

80 

330 

384 

433 

501 

85 

342 

397 

446 

5i6 

90 

353 

410 

459 

530 

95 

364 

422 

472 

543 

TOO 

375 

435 

484 

556 

"5 

425 

486 

540 

617 

ISO 

468 

532 

588 

669 

175 

507 

574 

633 

717 

2OO 

542 

612 

672 

781 

CHAPTER  V. 

THE  INDICATOR  ON  THE  AIR-COMPRESSOR. 
READING    AND   COMPUTING   THE   DIAGRAM. 

THE  recent  advances  that  have  been  made  in  steam- 
engine  economy  are  not  fully  and  generally  realized.  The 
engines  of  the  Great  Eastern  steamship  of  forty  years  ago, 
representing  the  best  engineering  practice  of  her  day,  de- 
veloped 8000  horse-power.  The  engines  of  the  Campania 
to-day  show  30,000  horse-power  upon  practically  the  same 
consumption  of  coal.  The  gain  is  attributable  to  the 
adoption  of  the  multiple  expansion-engine,  to  the  reheating 
between  the  steps,  and  to  the  general  prevention  of  con- 
densation  ;  but  the  promoter  and  adviser  all  along  the  way 
through  the  successive  stages  of  improvement  has  been  the 
indicator.  The  indicator  is  to-day  the  companion  and  the 
trusted  monitor  of  the  steam-engine  designer  and  builder 
as  well  as  of  the  engineer.  He  would  be  a  strange  competitor 
for  steam-engine  trade  in  these  days  who  would  not  freely 
and  gladly  show  the  cards  from  his  engine,  and  it  goes 
without  saying  that  he  would  be  an  unsuccessful  one. 
The  air-compressor  business  is  still  an  "  infant  industry," 
although  a  growing  one.  No  better  evidence  is  needed  of 
the  juvenility  of  the  air-compressor  trade  of  to-day  than 
the  difficulty  of  obtaining  cards  from  most  of  the 

39 


4O  COMPRESSED   AIR. 

"  standard  "  compressors.  And  yet  the  services  of  the 
indicator  may  be  as  valuable  to  the  air-compressor  and  the 
air-engine  as  to  the  steam-engine,  and  they  are  certainly 
fully  as  applicable. 

All  circumstances  seem  peculiarly  to  invite  the  applica- 
tion of  the  indicator  to  the  air-compressor,  and  to  the  study 
of  air-compression  practice  and  results  by  its  aid.  In  fact, 
the  air-compressor  seems  to  be  the  ideal  and  only  perfect 
field  for  the  indicator.  So  far  as  I  know,  a  steam-actuated 
air-compressor  is  the  only  machine  where  an  indicator  can 
be  applied  and  be  made  to  tell  the  whole  story  of  the 
power  developed  and  of  the  work  done.  In  the  steam- 
pump  the  report  of  the  card  of  the  water-cylinder  is  af- 
fected by  questions  relating  to  the  inertia  of  the  body  of 
water.  With  a  steam-engine  of  any  type  there  is  always 
some  uncertainty  about  the  friction  of  the  working  parts  of 
the  engine.  We  can  take  what  we  are  pleased  to  call  the 
"friction  diagram,"  when  the  engine  is  running  without 
doing  any  external  work,  and  we  know  what  resistance  the 
steam  has  to  overcome  at  that  time  ;  but  that  tells  us  com- 
paratively little  of  the  resistance  of  the  engine  parts  when 
loaded.  We  know  that  the  friction  of  nearly  every  work- 
ing part  of  the  engine  increases  with  the  load,  but  when 
the  load  is  on,  we  do  not  know  from  the  indicator-card  how 
much  of  its  mean  effective  indicates  actual  work  done  or 
how  much  of  it  belongs  to  the  friction  of  the  engine,  and 
to  get  the  result  with  any  certainty  and  accuracy  it  is  nec- 
essary to  employ  some  form  of  dynamometer  in  connection 
with  the  indicator,  and  let  them  fight  it  out  between  them. 
In  the  case  of  the  air-compressor  this  is  all  different.  The 
air-compressor  is  its  own  dynamometer.  By  taking  cards 
from  both  the  air-  and  the  steam-cylinders  at  the  same 


THE   INDICATOR   ON    THE   AIR-COMPRESSOR.     4* 

time,  or  when  the  compressor  is  running  under  the  same 
conditions,  we  get  a  perfect  statement  of  the  power  de- 
veloped and  of  the  actual  work  done,  and  then  we  know 
too  that  the  difference  in  indicated  horse-power  between 
the  air-  and  the  steam-cards  clearly  shows  the  power  that 
has  been  expended  merely  to  keep  the  machine  in  motion. 
The  cards  not  only  give  the  comparative  total  power  and 
work,  but  also  the  relations  of  the  one  to  the  other  at  any 
point  of  the  stroke,  showing  the  air  resistance  at  any  point, 
as  well  as  the  force  of  the  steam  at  the  same  point,  and 
through  this  knowledge  it  will  advise  us  whether  the  air  is 
compressed  with  economy  or  whether  better  results  are  to 
be  sought  for. 

Realizing  the  importance  of  the  indicator  as  an  indis- 
pensable aid  in  the  full  development  of  economical  air- 
compression,  it  is  proper  that  we  learn  what  we  can  of  the 
peculiarities  of  the  air-card  and  of  the  means  of  manipulat- 
ing and  interpreting  it.  We  can  only  consider  at  first  the 
card  from  the  single  air-cylinder,  in  which  the  whole  opera- 
tion of  air-compression  is  completed  at  a  single  stroke. 
The  cards  from  cylinders  in  which  either  stage  of  a  com- 
pound compression  is  carried  on  assume  peculiar  shapes, 
which  we  may  find  pleasure  in  studying  later  on. 

To  an  indicator-man  who  has  been  brought  up,  as  most 
have,  exclusively  upon  steam-cards  the  air-card  is  at  first  a 
little  confusing,  from  the  fact  that  all  the  operations  upon 
the  one  card  are  the  reverse  of  those  upon  the  other.  The 
admission-line  of  the  steam-card  is  the  delivery-line  of  the 
air-card  ;  the  expansion-line  in  the  one  is  the  compression- 
line  in  the  other  ;  the  exhaust  or  back-pressure  line  is  the 
admission-line,  and  the  compression-line  becomes  the  re- 
expansion-line.  One  can,  however,  soon  "  catch  on  "  and 


42  COMPRESSED   ATR. 

become  familiar  with  each  operation  and  its  representative 
part  of  the  diagram. 

It  is  not  the  purpose  of  this  work  to  instruct  in  the  ap- 
plication and  use  of  the  indicator.  We  must  assume  that 
the  indicator  is  in  competent  hands,  or  its  evidence  will  be 
worthless.  Indicator-cards  have,  however,  a  way  of  telling 
for  themselves  frequently  if  they  have  not  been  taken  with 
a  reasonable  regard  for  the  essential  conditions.  As  the 
peculiarly  important  part  of  the  air-card  is  the  compression- 
line,  it  is  necessary  that  the  drum  movement  be  correct,  and 
that,  in  proportion  to  its  length,  the  travel  of  the  card  shall 
be  accurately  coincident  with  the  piston  travel  at  all  points. 
Cards,  to  be  relied  upon,  should  not  be  taken  until  the  com- 
pressor has  been  run  long  enough  to  have  attained  its  com- 
plete working  conditions.  We  know  that  the  compression 
of  air  heats  it,  and  that  the  heat  then  in  the  air  is  commu- 
nicated more  or  less  to  everything  in  contact  with  it. 
When  the  cylinder  becomes  heated,  it  has  its  effect  back 
again  upon  the  air,  and  until  the  compressor  has  been  run 
continuously  and  at  full  pressure  for  an  hour  or  so,  the  full 
temperature  of  the  working  parts  has  hardly  been  reached, 
and  the  effect  of  the  heated  parts  upon  the  temperature  of 
the  air  at  different  points  of  the  stroke  will  not  be  correctly 
indicated.  Cards  taken  from  a  compressor  that  has  only 
just  been  started  will  give  a  lower  compression-line  and  a 
lower  mean  effective  pressure  (M.E.P.)  than  those  taken 
after  the  cylinder  and  piston  and  connecting  parts  have 
been  heated  up  to  their  mean  working  temperature. 

Fig.  3  is  offered  as  an  ideal  and  typical  single-compres- 
sion air-cylinder  card,  designed  to  show  the  points  and 
properties  of  the  card,  and  the  methods  of  manipulating 
and  studying  it.  The  card  is  somewhat  smoother  and 
cleaner  and  in  most  respects  more  perfect  than  any  actual 


THE   INDICATOR    ON    THE  AIR-COMPRESSOR.     43 


card,  except   that   the   admission-line   is   purposely  drawn 
rather  low  to  keep  it  perfectly  distinct  from  the  atmosphere- 

ta 


line.     The  lines  constituting   the   actual   diagram   are 
follows : 

AB,  Compression-line 

BQ,  Delivery  " 

CD,  Re-expansion  " 

DA,  Admission        " 


44  COMPRESSED   AIR. 

These  constitute  the  actual  card,  and  together  represent 
the  complete  cycle  of  operations  occurring  in  one  end  of 
the  air-cylinder  for  one  complete  revolution  of  the  com- 
pressor-crank. The  atmosphere-line,  MN,  is  also  traced 
by  the  indicator,  and  is  the  neutral  line  of  the  diagram,  or 
the  line  of  departure  in  air-compression. 

For  the  proper  interpretation  of  the  diagram  additional 
lines  are  to  be  drawn  as  follows  :  EF,  the  line  of  perfect 
vacuum.  This  line  is  drawn  parallel  to  the  atmosphere- 
line,  MN,  and  at  a  distance  below  it  determined  by  the 
scale  of  the  diagram.  The  pressure  of  the  atmosphere  at 
sea-level  being  14.7,  and  always  decreasing  as  the  altitude 
increases,  the  practice  of  calling  the  atmospheric  pressure 
15  Ibs.  may  be  said  to  be  a  rather  loose  one.  If  the  com- 
pressor is  operated  at  a  considerable  altitude  above  the 
sea-level,  as  many  are,  the  atmospheric  pressure  at  the 
time  and  place  where  the  diagram  is  taken  should  be  ascer 
tained  by  a  barometer,  and  the  line  EF  be  drawn  accord- 
ingly. It  should  be  remembered,  as  we  will  see  when  we 
get  to  it,  that  a  height  of  only  a  quarter  of  a  mile,  or  a  little 
over  1300  feet,  will  make  a  difference  of  7  per  cent  in  the 
volume  of  air  furnished. 

The  vertical  lines  PA  and  CL  having  been  drawn  per- 
pendicular to  MN,  and  defining  the  extreme  length  of  the 
actual  diagram,  the  clearance-line  GJ?  may  next  be  drawn. 
This  is  drawn  parallel  to  CL,  and  the  distance  CG  or  LH 
may  be  ascertained  by  computation.  The  volume  repre- 
sented by  the  rectangle  APCL  is  the  actual  displacement  of 
the  piston  for  its  whole  travel.  The  volume  of  air  acted 
upon  by  the  piston  is  this  volume  increased  by  the  volume 
CGHL  remaining  in  the  clearance-space  of  the  cylinder. 
This  volume  of  air,  CGHL,  at  the  end  of  the  compression- 
stroke,  and  at  the  pressure  indicated  by  the  diagram,  has  upon 


THE   INDICATOR   ON    THE  AIR-COMPRESSOR.     45 

the  return  stroke  of  the  piston  re-expanded  until  it  reached 
the  atmospheric  pressure  again  at  D.  This  re-expansion  is 
so  quickly  accomplished  that  whatever  the  temperature  at  the 
beginning  the  re-expansion  is  practically  adiabatic.  The 
relative  volume  before  and  after  the  re-expansion  may  be 
found  in  column  5  of  Table  IV.  Assuming  the  scale  of  the 
diagram  to  be  30  and  the  pressure  at  CG  to  be  70  Ibs. 
gauge,  and  designating  LH  by  x  we  have  the  proportion  , 

x  :  DL  +  x  :  :  .288  :  i 
Then  the  length  DL  being  .25",  we  have 

x  :  .25  +  x  :  :  .288  :  i ; 
then 

x=  .072  +.288*, 
and 

.712*  =  .072, 
x  =  .101. 

So  that  CG  or  LH  equals  say  -fa",  and  GH  may  be  drawn 
accordingly. 

Having  drawn  GH,  the  rectangle  APGH  represents  the 
total  volume  of  air  subjected  to  compression  for  the  stroke, 
and  noting  the  point  a,  at  which  the  compression-line  be- 
gins to  rise  from  the  atmosphere-line,  and  drawing  the 
perpendicular  ae,  then  aeGH  represents  the  total  volume 
of  air  at  atmospheric  pressure.  The  point  a,  being  the 
point  at  which  compression  from  atmospheric  pressure 
begins,  may  be  considered  the  beginning  of  the  whole 
diagram,  and  the  cycle  of  operations  for  the  entire  stroke 
may  be  considered  to  start  from  this  point. 

For  computing  the  mean  effective  resistance  the  entire 
enclosed  area  of  the  actual  diagram  ABCD  is  to  be  taken, 


46  COMPRESSED   AIR, 

and  this  area  may  be  measured  by  the  planimeter,  or  by  the 
mean  of  a  series  of  ordinates  in  the  customary  way,  as  with 
any  other  diagram.  The  area  lying  below  the  atmosphere- 
line  of  course  represents  the  resistance  upon  the  return 
stroke,  but  the  diagrams  from  both  ends  of  the  cylinder 
being  assumed  to  be  similar,  the  entire  area  may  be  taken 
for  the  single  stroke.  The  correct  practice  is  to  take  dia- 
grams from  both  ends  of  the  cylinder,  and  it  should  be 
followed  if  possible,  but  it  is  clearer  and  simpler  for  us 
here  to  consider  only  the  single  diagram. 

The  M.E.P.  of  the  diagram  having  been  ascertained,  the 
indicated  horse-power  (I.H.-P.)  represented  may  be  com- 
puted precisely  as  in  the  case  of  a  steam-engine.  Thus  the 
M.E.P.  in  the  diagram  before  us  happening  to  be  30,  if  it 
were  taken  from  a  cylinder  20"  dia.  X  24"  stroke  at  80 
revolutions  per  minute,  the  I.H.-P.  for  the  double  stroke 
will  be  as  follows  : 

2o2  X  .7854  X  30  X  4  X  80  -~  33,000. 

I  like  always  in  such  cases  to  put  it  down  in  this  way,  that 
I  may  be  sure  that  I  get  in  all  the  ingredients.  It  is  not 
necessary  to  run  for  a  table  of  squares  or  of  areas,  and  no 
time  is  saved  by  doing  so.  The  decimal  .7854  is  always 
cleanly  divisible  by  the  constant  divisor  33,000,  giving  us 
.0000238.  It  is  not  difficult  to  remember  this  or  to  keep  it 
posted  with  other  labor-saving  devices  in  a  convenient  place. 
The  ciphers  in  the  other  factors  will  help  us  to  elbow  the 
decimal  point  to  the  right,  and  our  case  will  then  stand  like 
this,  a  little  string  of  simple  and  easy  multiplications  : 

2*  X  .238  X  3  X  4  X  8  = 

.238  X  384  =  91.39  I.H.-P. 

We  will  not  here  go  into  the  question  of  the  additions  to 
be  made  to  this  for  friction,  etc. 


THE  INDICATOR   ON    THE  AIR-COMPRESSOR.     47 

The  I.H.-P.  having  been  ascertained,  that  gives  us  the 
power  consumed,  or  the  cost  of  the  compression,  and  then 
we  naturally  want  to  know  as  soon  as  possible  the  actual 
quantity  of  air  compressed  and  delivered,  or  how  much  we 
have  got  for  our  money.  The  indicator-diagram  shows 
this  very  accurately.  At  the  point  a,  where  the  compres- 
sion-line takes  its  departure  from  the  atmosphere-line,  the 
cylinder  is  shown  to  be  full  of  air  at  the  atmospheric  press- 
ure and  corresponding  density.  This  is  not  the  whole 
cylinder,  as  a  portion  of  it,  Aa,  has  been  already  traversed 
by  the  piston.  Whatever  proportional  distance  the  point 
a  may  be  from  the  beginning  of  the  stroke  is  to  be  deducted 
from  the  total  length'of  the  stroke  and  the  remainder  repre- 
sents the  total  actual  volume  of  air  at  atmospheric  pressure 
subjected  to  compression  for  that  stroke.  The  compres- 
sion and  delivery  of  the  air  goes  on  with  the  advance  of 
the  piston  until  it  reaches  the  extreme  end  of  its  stroke  at 
CL,  but  when  that  is  reached,  the  clearance-space  LCGH 
is  filled  with  air  compressed,  but  not  delivered,  and  upon 
the  return  of  the  piston  this  air  re-expands  until  it  reaches 
the  atmosphere-line  at  o,  so  that  practically  the  travel  of 
the  piston  from  o  to  L  and  back  again  has  accomplished 
nothing  toward  compression,  and  the  distance  oL  is  also 
to  be  deducted  from  the  total  length  of  the  line  AL,  when 
that  line  is  taken  to  represent  the  volume  of  air  compressed 
and  delivered.  In  the  diagram  before  us  if  AL  be  3$"  and 
ao  be  3yV">  the  ratio  of  air  compressed  and  delivered  is 

^  =  88  per  cent  of  the  cylinder  capacity.     As  was  re- 
3-°75 

marked,  this  diagram  does  not  represent  actual  practice, 
and  the  ratio  is  not  usually  as  low  as  this,  the  loss  being 
more  frequently  found  hovering  about  5  per  cent  in  the 
best  compressors,  and  rarely  below  that. 


48  COMPRESSED   AIR. 

So  far  as  the  indicator  has  anything  to  say  about  the 
economy  of  the  air-compression, — and  it  has  much  to  say, 
— its  evidence  is  found  chiefly  in  the  compression-line  of 
the  diagram,  and  for  comparison  it  is  necessary  to  describe 
upon  the  diagram  the  theoretical  isothermal  and  adiabatic 


curves.  To  facilitate  the  drawing  of  these  lines  the  dia- 
grams Figs.  4  and  5  have  been  provided.  The  dimensions 
of  the  book  have  made  it  necessary  to  engrave  these  at  one 
half  of  the  full  size.  They  can  readily  be  reproduced  in 
full  size  by  any  draughtsman,  and  will  be  found  useful  for 
the  purpose  for  which  they  were  designed.  As  they  stand 
here  they  are  correct  for  scales  that  are  double  those  indi- 


THE  INDICATOR   ON   THE  AIR-COMPRESSOR.      49 

cated.  Thus  the  15  ordinate  is  correct  to  apply  to  a  30- 
scale  diagram,  the  20  ordinate  for  a  4o-scale  diagram,  etc. 
The  compression-line  of  the  air-card  is  more  easily  studied 
than  the  expansion-line  of  the  steam-card,  as  it  always  has  a 
definite  beginning  or  point  of  departure  at  a,  such  as  the 


Adiabatic   Compression 
Fiff.5 

steam-card  never  has.  From  this  point  a  the  isothermal  and 
the  adiabatic  curves  are  to  be  drawn.  When  the  compressing 
piston  is  at  ae,  the  air  under  compression  includes  the  con- 
tents of  the  clearance-space  at  the  farther  end  of  the 
cylinder,  and  the  total  body  of  air  under  compression  is 
represented  by  the  rectangle  aeGH.  Vertical  lines  then 


SO  COMPRESSED   AIR. 

are  to  be  drawn  dividing  this  space  into  20  equal  sections, 
and  for  convenience  the  lines  are  to  be  numbered,  begin- 
ning at  the  line  next  to  ae,  19,  18,  17,  16,  etc.  It  will  not 
be  necessary  to  number  the  last  two  or  three  lines  to  the 
right,  or  even  to  draw  them,  as  the  curves  will  not  reach 
them.  It  will  be  noticed  that  the  numbering  does  not 
include  the  boundary-lines  ae  and  GH.  Referring  now  to 
the  diagram  Fig.  5,  for  drawing  the  adiabatic  curve,  AB 
is  the  atmosphere-line  and  CD  is  the  line  of  perfect 
vacuum,  or  the  zero  line  of  absolute  pressure.  Taking 
from  the  diagram,  Fig.  5,  the  ordinate  line  correspond- 
ing to  the  scale  of  the  indicator-card,  the  distance  between 
AB  and  CD,  measured  upon  this  line,  is  the  distance  be- 
tween the  atmosphere-line  and  the  line  of  perfect  vacuum, 
and  the  vacuum-line  may  be  drawn  upon  the  card  accord- 
ingly parallel  to  the  atmosphere-line  and  at  this  distance 
from  it.  Then  upon  the  same  vertical  line  of  the  diagram 
Fig.  5  the  distance  from  AB  to  the  first  intersecting  line 
above  it  indicates  the  distance  to  be  laid  off  upon  the  ver- 
tical line  No.  19  of  the  card  as  one  point  of  the  required 
adiabatic  curve.  The  distance  from  AB  to  the  second  line 
above  is  the  distance  to  be  laid  off  upon  the  vertical  line 
No.  1 8  as  another  point  of  the  required  curve,  and  so  on  : 
the  points  may  be  successively  laid  off  upon  the  vertical 
lines  of  the  card  until  the  delivery-line  BC  is  reached,  or  a 
little  above  it,  when  it  is  unnecessary  to  go  further,  and  the 
required  curve  may  be  drawn  coincident  with  the  points  that 
have  been  thus  located.  The  isothermal  curve  may  be  drawn 
in  the  same  way  by  the  aid  of  Fig.  4.  The  points  upon  the 
first  two  or  three  vertical  lines  to  the  left  may  be  so  close  to 
the  actual  compression-line  of  the  card,  or  so  nearly  coinci- 
dent with  it,  that  it  is  more  confusing  than  helpful  to  draw 
the  lines,  and  they  may  begin  at  a  point  further  along  the  line. 


THE  INDICATOR   ON    THE  AIR-COMPRESSOR.      51 

This  diagram,  Fig.  5,  assumes  the  atmospheric  pressure  to 
be  14.7  Ibs.,  and  is  only  applicable  for  approximately  that 
pressure.  If  the  atmospheric  pressure  for  the  altitude  at 
which  the  compressor  works,  and  where  the  indicator-card 
was  taken,  is  decidedly  less  than  14.7,  the  atmosphere-line 
drawn  by  the  indicator  will  represent  that  pressure  and  will 
not  represent  14.7  Ibs.  The  mean  effective  pressure  can  or 
course  be  computed  by  taking  the  area  of  the  indicator- 
card  as  it  stands;  but  if  it  is  desired  to  draw  the  adiabatic 
and  isothermal  curves  by  the  aid  of  our  diagrams,  Figs.  4 
and  5,  it  will  be  necessary  to  first  draw  a  horizontal  line 
representing  the  atmospheric  pressure  of  14.7  Ibs.  To  do 
this  first  draw  the  zero  line  at  a  distance  below  the  existing 
atmosphere-line  corresponding  with  the  ascertained  atmo- 
spheric pressure  and  the  scale  of  the  diagram.  Column  i 
or  2  in  connection  with  column  4  of  Table  IV.,  given  at 
the  end  of  this  chapter,  will  generally  furnish  the  data 
necessary  for  this  service.  The  zero  line  having  been 
drawn  the  sea-level  atmosphere-line  may  then  be  drawn 
14.7  Ibs.  above  according  to  the  indicalor-scale.  When  this 
line  is  drawn  the  point  where  the  compression-curve  crosses 
it  may  be  noted  and  also  the  point  where  the  reexpan- 
sion  line  strikes  it,  and  ignoring  the  original  atmosphere- 
)ine  drawn  by  the  indicator,  the  adiabatic  and  the  isother- 
mal curves  may  be  drawn  precisely  as  previously  discribed. 

These  adiabatic  and  isothermal  curves  when  described 
are  rather  an  aid  to  the  eye  in  making  comparisons  with 
the  actual  compression-line  of  the  indicator-card  than  nec- 
essary in  computation.  The  mean  effective  of  the  card  is 
ascertained  by  the  planimeter  or  by  measurement,  and  the 
mean  effective  for  adiabatic  and  isothermal  compression  un- 
der the  same  conditions  may  be  found  in  Table  II,  and  the 
economy  of  the  actual  comnression  mav  be  learned  by  com- 


COMPRESSED   AIR. 


parison  with  them.     This  paragraph  is  only  meant  to  apply 
to  approximately  sea-level  computations. 

Table  IV  will  be  found  convenient  in  computations  upon 
air-compression  at  various  heights  above  the  sea-level. 
Column  7  gives  the  values  of  the  volumes  of  air  actually 
compressed  at  any  given  height  as  compared  with  equal 
volumes  of  free  air  at  sea-level. 

TABLE  IV. 

TABLE  OF  ABSOLUTE  PRESSURES,   BOILING-POINTS,  ETC.,  AT  DIFFERENT 
HEIGHTS  ABOVE   SEA-LEVEL. 


I 

9 

3 

4 

5 

6 

7 

Weight 

Volume  of 

Volume  of 

Height 
above 
Sea-level, 
Feet. 

Bar- 
ometer, 
Inches  of 
Mercury. 

Boiling- 
point, 
Degrees 
Fahr. 

Absolute 
Pressure, 
Lbs. 

Of  I 

Cu.  Ft. 
of  Air  at 
60°, 
Lbs. 

A      Equal 
to     Cu.  Ft. 
o    Free 
ir  at 
Se  -level. 

Free  Air  at 
Sea-level 
equal  to  iCu. 
Ft.  at  given 
Altitude. 

o 

30 

212 

14.7 

.0765 

r 

I 

5" 

29.42 

211 

14.41 

.07499 

.02 

.98039 

1025 

28.85 

210 

14-136 

.07356 

.04 

.96154 

1539 

28.29 

209 

13.86 

.07213 

.06 

•9434 

2063 

27-73 

208 

I3.587 

.07071 

.08 

.9259 

2589 

27.18 

207 

13.318 

.0693 

.IO 

.90909 

3"5 

26.64 

206 

I3.054 

.06793 

.12 

.89285 

3642 

26.11 

205 

12.794 

.06658 

.14 

.87719 

4169 

25-59 

204 

12.539 

.06525 

•17 

.8547 

4697 

25.08 

203 

12.289 

.06395 

.19 

.8403 

5225 

24.58 

202 

12.044 

.06267 

.22 

.8197 

5764 

24.08 

201 

11.799 

.0614 

•24" 

.8064 

6304 

23-59 

200 

11-559 

.06015 

•27 

.7874 

6843 

23.11 

199 

11.324 

•05893 

.29 

•  7752 

738i 

22.64 

198 

11.094 

•05773 

•32 

•75757 

7932 

22.17 

197 

10.863 

•0565 

•35 

•74074 

8481 

21.71 

I96 

10.638 

•05536 

•38 

.7246 

9031 

21  .26 

195 

10.417 

•05421 

.41 

.7092 

9579 

2O.82 

194 

IO.2O2 

•05309 

•44 

.6944 

10127 

20.39 

193 

9-99 

.05199 

•47 

.6802 

10685 

19.96 

192 

9.78 

.0509 

•50 

.6666 

11243 

19.54 

I9I 

9-57 

.0498 

•53 

.6536 

11799 

I9.I3 

9-37 

.0488 

•  56 

.  64  i  02 

12367 

18.72 

9.17 

•0477 

.60 

.625 

"934 

18.32 

8.98 

.0467 

•63 

•6135 

13498 

17-93 

I87 

8.78 

•0457 

.67 

.6 

14075 

17-54 

1  86 

8.59 

.0447 

•  71 

.5848 

14649 

I7.I6 

185 

8.41 

•0437 

•74 

•5747 

CHAPTER  VI. 
THE  BEGINNING  OF  ECONOMICAL  AIR-COMPRESSION. 

WHILE  it  may  easily  appear  that  the  purpose  for  which 
compressed  air  is  used  in  any  given  case,  or  the  conditions 
under  which  it  is  applied,  make  the  question  of  power 
economy  a  distinctly  subordinate  one,  and  often  relatively 
a  very  little  and  unimportant  one,  still  it  remains  that  for 
whatever  purpose  we  use  the  air  the  cheaper  we  get  it  the 
better  it  is  for  us,  and  considerations  of  economy  in  the 
compression  of  it  are  always  in  order,  and  whatever  saving 
is  effected  there  is  necessarily  a  clear  gain. 

If  we  are  to  go  into  the  compressed-air  business  "  for  all 
there  is  in  it,"  the  way  to  do  it  successfully  and  profitably 
is  first  of  all  to  control  the  whole  business.  By  this  is  not 
meant  the  establishment  of  a  monopoly  whereby  we  might 
have  the  compressing  of  all  the  air  that  is  used,-although 
there  might  be  great  profit  in  that.  But  with  what  air  we 
do  handle  we  cannot  expect  to  accomplish  much  in  the 
way  of  economy,  unless  we  have  as  full  control  as  possible 
of  the  air  through  each  of  the  operations  involved  in  its 
use  :  the  compression  of  the  air,  its  transmission  to  the 
place  or  to  the  apparatus  where  it  is  to  be  used,  and  its 
actual  employment  for  the  purpose  intended.  There  are 
losses  possible  at  several  points,  or  at  all  points,  along  the 
series  of  operations,  and  there  are  commensurate  savings  to 

53 


54  COMPRESSED   AIR. 

be  effected  by  the  avoidance  of  those  losses.  The  losses 
may  be  defeated  and  the  savings  accomplished  rather  by  a 
concentrated  than  by  a  divided  control  and  accountability. 

Economy  in  air-compression  should  begin  at  the  begin- 
ning, and  at  the  beginning  we  first  have  to  do  with  the 
''  free  air,"  or  air  at  atmospheric  pressure.  This  is  our  raw 
material,  and  it  is  of  course  desirable  to  get  it  as  cheaply 
as  possible.  Now  it  so  happens  that  in  keeping  our  ac- 
counts of  profit  and  loss  in  this  business  the  raw  material 
is  measured  out  to  us  and  charged  against  us  not  by 
weight,  but  by  bulk,  so  that  whatever  air  we  want  to  use  it 
is  desirable  to  get  it  to  the  compressor  in  as  small  a  volume 
as  possible.  The  smaller  the  relative  volume  of  air  at  the 
beginning  of  the  series  of  operations  the  greater  will  be  the 
profit  at  the  end  for  any  given  service  realized.  The  vol- 
ume of  free  air  increases  or  diminishes  as  its  temperature 
rises  or  falls,  which  means  that  we  should  get  our  free  air 
as  cold  as  possible.  The  colder  the  air  is  the  less  it  will 
measure  in  cubic  feet,  and  we  may  consequently  say  that 
the  colder  it  is  the  cheaper  we  are  getting  it. 

It  seems  necessary  in  all  of  the  operations  with  com- 
pressed air  to  keep  the  accounts  of  profit  and  loss,  and  the 
record  of  work  done,  by  the  volume  of  free  air  that  is 
handled.  This  involves  fewer  uncertainties  than  if  we 
were  to  base  our  computations  upon  the  volume  of  air  after 
compression  to  any  given  pressure,  or  at  any  later  stage  in 
its  transmission  or  use.  The  air-compressor,  when  the 
necessary  corrections  for  clearance,  etc.,  have  been  deter- 
mined, is  a  very  reliable  air-meter,  and  from  it  may  be  ob- 
tained a  very  close  record  of  the  free  air  taken  in  by  it  and 
compressed  and  delivered.  After  the  beginning  of  opera- 
tions the  temperature  of  the  air  is  such  an  uncertain  and 


BEGINNING    OF  ECONOMICAL   AIR-COMPRESSION.    55 

variable  factor,  and  is  still  of  such  importance  in  the  result, 
that  all  calculations  are  upset  by  it.  The  absolute  meas- 
ure of  the  air  operated  upon  would  of  course  be  its  weight, 
but  this  it  is  not  possible  to  ascertain  in  extensive  practical 
operations. 

The  volume  of  air  at  common  temperatures  varies  di- 
rectly as  the  absolute  temperature.  With  our  air-supply  at 
60°  its  absolute  temperature  is  521,°  and  the  volume  of  it 
will  increase  or  decrease  ?^T  for  each  degree  of  rise  or  fall 
of  temperature.  In  securing  our  supply  of  free  air  for  the 
compressor,  then,  if  we  can  get  a  difference  in  our  favor  of 
5°  by  laying  a  pipe  and  leading  the  air  in  from  the  outside 
of  the  compressor-room,  or  from  the  shady  side  of  the 
building,  or  from  the  coolest  place  near,  by,  instead  of 
using  the  air  in  the  compressor-room,  we  accomplish  a  sav- 
ing of  about  i  per  cent.  If  we  secure  a  difference  of 
temperature  of  10°,  which  in  practice  is  frequently  quite 
possible,  we  save  2  per  cent  absolutely  without  cost,  ex- 
cept the  first  cost  of  the  pipe  or  box  to  lead  the  air  in.  I 
know  that  the  average  machinist  or  engineer,  or  the  man 
who  calls  himself  distinctively  the  practical  man,  cannot 
commonly  appreciate  these  small  figures,  or  have  any  re- 
spect for  such  small  savings,  but  when  it  comes  to  business 
I  do  not  know  why  they  should  not  have  the  same  weight 
as  the  same  values  have  in  any  other  of  the  details  of  busi- 
ness. Brokers  have  to  live  and  flourish  upon  commissions 
of  ^  or  -j^  of  i  per  cent,  but  "  practical "  men  are  so 
wealthy  that  i  or  2  per  cent  is  not  worth  considering. 

The  pipe  to  convey  the  cool,  free  air  from  the  point 
where  we  determine  to  take  it  to  the  compressor  may  as 
well  be  of  wood  or  of  cement  or  earthenware  as  of  iron, 
and  in  fact  such  material  for  its  non-conductivity  is  to  be 


56  COMPRESSED    AIR. 

preferred.  The  pipe  should  of  course  be  large  enough  to 
convey  the  required  flow  of  air  with  perfect  freedom. 
Some  of  the  best  air-compressors  of  the  day  may  be  con- 
nected quite  readily  with  an  outside  air-supply,  and  they 
make  provision  for  it  ;  others  cannot  easily  be  so  con- 
nected, which  is  unfortunate  for  them. 

Another  point  that  has  not  hitherto  received  the  atten- 
tion that  it  deserves,  although  much  more  important  than 
the  preceding,  is  the  necessity,  in  the  interest  of  the  best 
power  economy,  of  not  only  getting  the  air  as  cold  as  pos- 
sible at  the  compressor,  but  of  getting  it  as  cold  as  possible 
into  the  compressor.  We  have  too  readily  assumed  that 
the  one  covers  the  other,  when,  as  a  matter  of  fact,  it  never 
does.  The  temperature  of  the  air  at  the  cylinder  and  about 
to  enter  it  does  not  guarantee  the  temperature  of  the  air  in 
the  cylinder  at  the  moment  when  the  cylinder  is  filled  and 
compression  begins.  It  is  not  too  much  to  say  that  the 
temperature  of  the  air  outside  the  cylinder  and  of  that 
inside  is  never  the  same.  Yet  it  is  not  to  be  forgotten  that 
the  sole  object  of  the  effort  to  get  cool  air  for  the  compres- 
sor is  to  have  it  as  cool  as  possible,  and  of  as  small  a  volume 
as  possible,  at  the  moment  when  compression  begins.  How 
cool  the  air  may  have  been  at  any  previoas  moment,  how- 
ever near,  has  nothing  to  do  with  the  case. 

In  another  chapter  I  have  remarked  that  the  air-compres- 
sor is  the  ideal  and  the  only  perfect  field  for  the  use  of  the 
indicator,  that  it  is  the  only  place  where  the  indicator  dia- 
grams will  tell  the  whole  story  both  of  the  power  expended 
and  of  the  work  accomplished.  This  is  undoubtedly  true, 
but  it  is  a  statement  that  is  quite  likely  to  be  understood  to 
say  more  than  it  does  say.  The  indicator  diagram  from 
the  air-cylinder  does  not  tell  all  that  it  seems  to  tell,  or  it 


BEGINNING    OF  ECONOMICAL   AIR-COMPRESSION.    57 

tells  it  wrong.  You  may  note  upon  the  diagram  the  point, 
very  near  the  beginning  of  the  compression-stroke,  where 
the  cylinder,  if  we  may  believe  the  diagram,  is  filled  with 
free  air,  or  air  at  atmospheric  pressure,  and  from  that,  after 
deducting  what  fills  the  clearance-space  at  the  end  of  the 
stroke,  we  may  compute  the  volume  of  free  air  actually 
compressed  and  delivered  ;  and  then,  later,  we  may  realize 
chat  we  have  not  got  the  volume  of  free  air  that  the  dia- 
gram testifies  to.  This  is  due  to  the  fact  that  the  diagram 
has  nothing  to  say' about  the  actual  temperature  of  the  air, 
either  at  its  admission,  at  its  discharge,  or  at  any  point  of 
the  stroke.  With  steam,  unless  it  is  superheated,  the  press- 
ure indicated  guarantees  the  temperature;  with  air  the 
pressure  and  the  temperature  have  no  necessary  connection. 
I  may  show  you  a  diagram  from  an  air-compressing  cylin- 
der where  the  air-admission  line  is  almost  exactly  coin- 
cident with  the  atmosphere  line,  and  where  the  compression 
line  begins  to  rise  above  the  atmosphere  line  immediately 
at  the  beginning  of  the  compression-stroke,  showing  that 
the  cylinder  is  completely  filled  with  air  at  atmospheric 
pressure,  and  we  may  congratulate  ourselves  that  the  dia- 
gram is  an  excellent  one  in  this  respect  ;  but  suppose  that 
when  the  cylinder  is  just  filled,  and  compression  is  just 
beginning,  our  cylinder  is  filled  with  air  at  120°  instead  of 
at  60°,  which  is  the  temperature  of  the  supply.  It  means 
that  our  cylinder  holds  rather  less  than  .9  of  the  air  that 
we  are  assuming  th'at  it  holds,  and  which  the  diagram  says 
that  it  holds.  It  means  not  merely  that  the  practical  capa- 
city of  the  compressor  is  one-tenth  less  than  we  assume  it 
to  be,  but  that  for  the  compression  of  this  nine-tenths  we 
are  still  expending  the  full  power  as  represented  by  the 
steam-card.  If  the  difference  in  indicated  horse-power 


58  COMPRESSED   AIR. 

between  the  air-cylinder  and  the  steam-cylinder  is  ten  per 
cent  of  the  air-cylinder,  or  if  the  power  ratio  of  the  steam 
to  the  air  be  i.i  :  i,  it  is  -not  a  bad  showing.  This  is  about 
the  ratio  obtained  in  the  best  air  compressors  of  the  day. 
But  if  this  i.i,  the  power  of  the  steam-cylinder,  is  to  be 
compared  not  with  i,  the  full  capacity  of  the  air-cylinder, 
but  with  .9,  its  actual  contents,  the  case  is  quite  different: 
.9  :  i.i  :  :  i  :  1.22,  which  is  a  result  not  worth  bragging 
about  by  any  compressor  builder. 

There  seems  to  be  no  means  of  ascertaining  the  actual 
temperature  of  the  air  during  the  operation  of  compression. 
The  temperature  of  the  air  at  different  points  of  the  stroke 
would  be  easily  computable  from  the  indicator  diagram, 
which  shows  the  pressure  attained  at  any  point,  if  we  only 
knew  the  initial  temperature,  but  as  we  have  no  means  of 
knowing  the  initial  temperature  we  do  not  know  the  actual 
temperature  at  any  time.  Who  will  tell  us  how  to  find 
it  out  ?  This  does  not  seem  to  be  an  impossible  problem. 
It  looks  at  first  sight  almost  as  simple — not  quite — as  to  tell 
how  fast  a  stream  of  water  flows  through  a  pipe.  But  no- 
body has  yet  invented  a  satisfactory  water-meter.  In  the 
meantime  we  can  only  use  our  mechanical  judgment  and 
common  sense  as  to  the  best  means  of  getting"  the  air  into 
the  cylinder  as  cool  as  possible.  We  can  say  in  a  general 
way  that  the  air  should  enter  the  cylinder  by  the  shortest 
and  most  direct  possible  passage,  and  with  as  little  contact 
as  possible  with  any  metal  at  a  higher  temperature  than  its 
own. 

Some  interesting  matter  bearing  upon  the  topic  we  are 
speaking  of  is  found  in  a  paper  upon  "  Blowing  Engines," 
by  Mr.  Julian  Kennedy  of  Pittsburgh,  read  before  the 
Mining  Engineering  Division  of  the  World's  Engineering 
Congress  at  Chicago.  Mr.  Kennedy  says: 


BEGINNING   OF  ECONOMICAL  AIR-COMPRESSION.    59 

"  This  heating  of  the  incoming  air  expands  it,  and  pro- 
portionately reduces  the  weight  of  air  entering  the  cylinder 
at  each  stroke.  I  have  observed  this  in  the  case  of  an 
engine  which  was  so  constructed  as  to  cause  the  air  to 
travel  about  3  inches  over  the  hot  metal  in  thin  films  TV 
thick.  Alongside  of  it  was  another  engine  of  the  same  size 
and  make,  except  that  valves  were  used  which  allowed  the 
air  to  pass  over  about  i  inch  of  metal,  the  openings  being 
of  such  size  that  each  stream  of  air  was  2  inches  in  thick- 
ness. Careful  and  repeated  tests  of  these  engines,  when 
both  were  in  good  order,  showed  that,  while  the  indicator 
diagrams  were  practically  the  same,  the  one  with  the  large 
valves  would  burn  about  loper  cent  more  coke  in  the  fur- 
naces, a  result  which  could  only  be  explained  on  the  sup- 
position that,  in  the  case  of  the  engine  with  the  small  air 
openings,  the  incoming  air,  in  passing  through  the  small 
and  tortuous  passages  in  the  heads,  was  heated  about  25°  C. 
more  than  in  the  case  of  the  other  engine." 

The  above,  it  should  be  remembered,  speaks  only  of  blow- 
ing engines,  where  the  air-pressures  are  low,  and  where  the 
heat  of  compression  and  the  heating  of  the  parts  in  contact 
with  the  compressed  air  do  not  range  high.  In  an  air-com- 
pressor every  part  of  the  cylinder  in  contact  with  the  air 
after  compression  naturally  becomes  much  hotter  than  in 
the  blowing-engines  that  Mr.  Kennedy  speaks  of,  and  the 
heating  of  the  inrushing  air  may  also  be  much  greater. 

The  air  remaining  in  the  clearance  space  of  the  air-cylin- 
der at  the  end  of  the  compression-stroke,  being  between 
the  hot  piston  and  the  more  or  less  heated  cylinder  head, 
may  not  have  lost  much  of  its  heat  of  compression,  but  by 
the  cooling  action  of  the  water-jacket  it  must  have  lost  some 
of  its  heat,  and  its  temperature  cannot  therefore  be  as  high 
as  the  theoretical  temperature  due  to  the  compression. 


60  COMPRESSED    AIR. 

Still  it  is  comparatively  hot,  and  when  it  is  remembered 
that  this  hot  air  becomes  a  part  of  the  next  cylinder  full  of 
air  to  be  compressed  it  has  been  assumed  that  therefore  the 
mean  temperature  of  the  contents  of  the  cylinder  is  some- 
what increased  by  this  admixture.  But  this  conclusion  is 
hasty  and  unwarranted.  This  hot  air  in  the  clearance- 
space  is  only  hot  when  under  the  terminal  pressure,  and  as 
at  this  pressure  it  is  not  as  hot  as  the  theoretical  tempera- 
ture for  the  given  compression  it  cannot  upon  its  re-expan- 
sion to  atmospheric  pressure  be  as  hot  as  it  was  before  its 
previous  compression  began.  It  must  be  really  somewhat 
cooler  than  the  air  that  rushes  in  to  fill  the  cylinder  for  the 
next  stroke,  and  it  therefore  does  not  contribute  any  heat  to 
the  new  charge  of  air,  but  rather  receives  some  heat  from  it 
and  slightly  cools  it. 

The  air  remaining  uncompressed  in  the  clearance  space 
at  the  end  of  the  compression  stroke,  as  it  does  not  raise 
the  temperature  of  the  incoming  air  or  tend  to  increase  its 
volume,  has  therefore  no  bad  effect  in  that  respect,  and  in 
no  way  increases  the  power  required  for  compressing  a 
given  quantity  of  air.  The  power  that  has  been  expended 
in  the  compression  of  this  air  in  the  clearance-space  is  not 
lost,  or  but  a  portion  of  it,  as  it  gives  out  in  its  re-expan- 
sion, by  helping  the  piston  upon  its  return  stroke,  most  of 
the  power  expended  in  its  compression.  Clearance  in  the 
air-cylinder,  therefore,  represents  a  loss  of  capacity  in  the 
air-compressor  rather  than  a  loss  of  power.  And  it  is  on 
account  of  its  reducing  the  capacity  of  the  compressor  to 
compress  its  full  quota  of  free  air  per  stroke  that  it  is 
desirable  to  keep  the  clearance  as  small  as  possible. 


CHAPTER   VII. 
OF   COMPRESSION   IN   A  SINGLE  CYLINDER. 

PROCEEDING  now  to  look  into  the  actual  conditions  of 
practical  air-compression,  and  the  possible  economy  to  be 
attained,  it  is  perhaps  most  proper  to  consider  the  perform- 
ance of  the  best  compressors  in  actual  use  rather  than  the 
ideal,  and  perhaps  in  some  respects  the  practically  impos- 
sible, compressor.  The  air-compressors  now  most  gener- 
ally in  use  have  horizontal,  double-acting  air-cylinders 
more  or  less  completely  water-jacketed,  and  with  various 
devices  for  heads,  valves,  pistons,  etc.  The  entire  com- 
pression is  effected  at  a  single  operation  and  the  pressure 
of  the  air  usually  ranges  from  60  to  80  Ibs.  gauge.  Whether 
these  compressors  prevail  through  the  operation  of  the  law 
of  natural  selection  and  the  survival  of  the  fittest  we  may 
not  rashly  say  ;  while  they  may  not  exhibit  the  highest 
attainable  economy  in  the  compression  they  are  found  to 
require  little  looking  after,  cost  little  for  repairs,  are  gener- 
ally reliable,  and  in  the  long  run  they  are  found  to  pay. 

Supposing  that  we  are  filling  the  air-cylinder  by  the 
natural  inflow  of  the  air  under  the  pressure  of  the  sur- 
rounding atmosphere,  and  that  we  have  got  into  the  cylin- 
der the  greatest  possible  actual  weight  or  quantity  of  air 
under  those  conditions,  which  means  that  our  air  is  little, 
if  any,  below  the  density  of  the  surrounding  air  from  which 

61 


62  COMPRESSED   AIR. 

it  is  drawn,  and,  assuming  that  the  air  is  also  as  cool  as  we 
can  get  it,  we  may  then  be  said  to  have  got  our  material  as 
cheaply  as  possible,  to  have  started  our  business  under  the 
most  favorable  conditions,  and  with  encouraging  prospects  ; 
and  we  may  then,  and  not  until  then,  consistently  and 
without  reproach  look  for  the  available  means  of  economy 
in  the  actual  operation  of  compression.  The  same  con- 
siderations that  tend  to  economy  in  the  procuring  of  the  airs 
or  of  getting  it  into  the  cylinder,  hold  good  also  in  all  the 
subsequent  operations  of  compression.  The  smaller  the 
bulk  or  volume  of  any  given  quantity  or  weight  of  air  the 
cheaper  can  the  compression  be  effected  and  the  better 
will  be  the  economy  ;  and,  as  the  volume  of  the  air  at  any 
given  pressure  depends  upon  its  temperature,  the  supreme 
consideration  throughout  the  operation  is  to  keep  the  air 
as  cool  as  possible.  The  question  of  temperature  is  the 
important  one  to  be  kept  constantly  in  sight,  and  its  im- 
portance resides  entirely  in  its  effect  upon  the  volume  of 
air  operated  upon.  While,  as  we  know,  practical  air-corn 
pression  has  not  as  yet  come  down  to  the  minute  econo- 
mies, where  eventually  the  profits  of  legitimate  business  are 
to  be  sought,  still  the  losses  that  are  possible  in  compres- 
sion, and  the  gains  that  are  to  be  effected  by  avoiding  or 
overcoming  those  losses,  have  received  more  or  less  atten- 
tion from  the  compressor  builders. 

It  is  well  enough  understood'  that,  in  the  interest  of 
power  economy,  the  air  should  be  kept  as  cool  as  possible 
at  every  stage  of  the  compression,  and  the  earlier  the  cool- 
ing is  effected  the  greater  is  the  gain,  as  all  of  the  subse- 
quent operation  is  more  or  less  affected  by  it.  Keeping 
the  air  cool  during  compression  means  actually  cooling  the 
air  during  compression.  No  compression  can  be  effected 


OF  COMPRESSION  IN  A    SINGLE   CYLINDER.      63 

without  a  corresponding  rise  of  temperature  in  the  air  com- 
pressed. Theoretically  the  rise  will  always  be  the  same 
where  the  conditions  are  identical.  Starting  with  a  given 
volume  of  air  and  with  the  air  at  a  given  pressure  and  tem- 
perature, and  compressing  to  another  and  higher  pressure, 
the  resulting  volume  and  temperature  should  always  be  the 
same.  Practically  the  temperature  of  the  air  after  com- 
pression, or  during  compression,  is  never  as  high  as  the 
theoretical  temperature,  or  as  high  as  the  books  and  tables 
say  that  it  should  be,  and  it  is  also  widely  variable  under 
apparently  slight  changes  of  conditions.  This  is  not  at  all 
because  the  theory  in  the  case  is  incorrect,  but  rather  that 
it  is  incomplete,  in  that  it  is  not  cognizant  of  all  the  condi- 
tions that  affect  the  case.  Theory  says,  and  correctly,  that 
the  element  of  time  has  nothing  to  do  with  the  heat  of 
compression ;  that  a  given  volume  of  air  when  compressed 
to  another  given  volume  will  have  its  temperature  raised  so 
much,  whether  it  takes  a  minute,  an  hour,  or  a  week  to  do 
it.  Practically  time  has  a  great  deal  to  do  with  the  case. 
The  readiness  with  which  the  air  will  receive  heat  from  or 
impart  it  to  whatever  may  be  in  contact  with  it,  and  the 
small  amount  of  heat  actually  represented  by  its  changes 
of  temperature  render  the  actual  volume  a  highly  elusive 
quantity,  and  time  becomes  a  playground  for  it. 

In  a  compressing-cylinder  in  actual  use  all  the  parts  of 
it,  the  body  of  the  cylinder,  the  heads,  the  piston  and  rod, 
the  valves  and  seats  or  guides  become  heated  by  their 
contact  with  the  compressed  air ;  but  while  they  are  thus 
becoming  heated  they  are  only  heated  by  this  contact,  and 
while  being  heated  they  are  also  being  cooled,  as  they  are 
constantly  transmitting  some  of  the  heat  received  from  the 
air  and  dispersing  it  by  conduction  or  radiation  ;  and,  con- 


64  COMPRESSED   AIR. 

sequently,  these  parts  are  never  as  hot  as  the  air  that  heats 
them — when  the  air  is  at  its  hottest — and  the  air  also  is 
not  as  hot  as  it  would  have  been  but  for  its  contact  with 
them.  The  metallic  parts  after  a  time  of  continuous  opera- 
tion attain  an  average  temperature,  and  will  not  get  any 
hotter.  The  mean  temperature  attained  will  depend  upon 
the  facilities  provided  for  taking  the  heat  away.  Nothing 
better  is  known  or  has  been  suggested  for  conveying  away 
the  heat  than  cold  water.  It  is  now  the  general  practice  to 
make  the  shell  of  the  cylinder  double  with  a  water-space 
between  the  cylinder  proper  and  the  outer  shell,  and,  where 
the  style  and  arrangement  of  the  valves  permit,  the  heads 
also  are  made  hollow,  with  water  circulating  in  them. 
Water  has  also  in  some  cases  been  circulated  in  the  body 
of  the  piston.  These  arrangements  undoubtedly  help  to 
reduce  the  mean  temperature  of  the  parts  and  to  make 
them  more  effective  in  cooling  the  air. 

When  the  entire  compression  is  effected  in  a  single 
cylinder  the  heat  of  compression  is  abstracted  from  the  air 
mostly  at  the  latter  part  of  each  stroke,  when  the  air  is  at 
its  hottest  and  when  the  difference  in  temperature  between 
the  air  and  its  surroundings  is  the  greatest.  Indeed  it  is 
to  be  supposed  that  in  active  compression  the  air  loses 
none  of  its  heat  of  compression  during  the  earlier  part  of 
the  stroke  unless  the  means  of  cooling  the  cylinder  parts 
are  unusually  efficient  and  operative.  If  at  the  beginning 
of  the  stroke  the  cylinder  is  hotter  than  the  air,  as  it  natu- 
rally must  be,  the  air  is  naturally  heated  rather  than  cooled 
by  the  contact.  Practical  evidence  of  this  is  not  wanting. 
Indicator  diagrams  from  air-compressing  cylinders  are  easily 
to  be  found,  as  Fig.  6,  where  the  compression-line  of  the 
diagram  does  not  leave  the  adiabatic  line  until  the  first 


OF  COMPRESSION  IN  A    SINGLE   CYLINDER.     6$ 

quarter  of  the  stroke  is  traversed.  In  this  connection  it 
may  be  remarked  that  for  evidence  upon  the  point  that  we 
are  considering  any  indicator-cards  that  are  taken  when  a 
compressor  has  just  been  started,  and  before  the  cylinder 


parts  have  attained  their  full  average  temperature,  are  not 
net  be  considered.  Such  cards  promise  better  than  the 
actual  performance  of  the  compressor  will  fulfil. 

The  heating  of  the  air  does  not  continue  throughout  the 
whole  stroke  of  the  piston,  but  is  accomplished  and  ceases 
at  the  moment  that  the  full  pressure  is  reached ;  and  for  the 


66  COMPRESSED   AIR. 

remainder  of  the  stroke,  while  the  compressed  air  is  being 
ejected  from  the  cylinder,  the  air  is  becoming  somewhat 
cooler,  while  the  metal  inclosing  it  is  becoming  hotter. 
The  heat  of  the  cylinder  parts  is  not  evenly  distributed. 
The  ends  of  the  cylinder  and  the  entire  cylinder-heads, 
being  exposed  to  the  air  when  it  is  hottest,  naturally  be- 
come hotter  than  the  middle  of  the  cylinder,  which  never 
feels  the  hottest  air.  The  importance  of  the  water-jacket,  in 
the  absence  of  any  better  cooling  device,  is  obvious  enough. 
The  cooling  effect  of  the  water  is  greater  when  it  is  applied 
to  the  cylinder-heads  than  anywhere  else,  because  they  are 
exposed  to  the  heated  air  for  the  greater  portion  of  the 
stroke,  while  the  inner  surface  of  the  cylinder  itself  is  cov- 
ered by  the  advancing  piston.  Apart  from  the  cooling  of 
the  air  under  compression,  and  the  reduction  of  its  volume, 
the  water-jacket  is  a  necessity  as  affecting  the  lubrication 
of  the  cylinder  surfaces.  Without  some  such  means  of 
cooling  the  cylinder  it  would  become  so  heated  as  to  burn 
the  oil  and  render  it  useless  as  a  lubricant. 

As  the  ultimate  object  of  the  water-jacket  is  the  saving 
of  power,  by  the  reduction  of  the  volume  of  air  under  com- 
pression, it  is  an  interesting  question  as  to  what  is  practi- 
cally accomplished  by  it.  What  cooling  of  the  air  is 
actually  effected  and  what  saving  of  power  is  accomplished 
by  complete  water-jacketing  ?  From  all  that  I  have  been 
able  to  observe  I  think  that  we  may  say  that  when  com- 
pressing in  a  single  cylinder  to  from  60  to  80  pounds  gauge- 
pressure,  and  at  a  piston-speed  not  exceeding  300  feet  per 
minute,  one  half  of  the  total  possible  cooling  is  all  that 
may  be  expected  to  be  accomplished.  This,  I  think,  may 
be  done,  although  I  will  not  undertake  at  this  writing  to 
show  where  such  a  performance  is  actually  to  be  found. 


OF  COMPRESSION  IN  A    SINGLE   CYLINDER.     67 

If  by  a  single  compression  we  can  produce  a  compression- 
line  midway  between  the  adiabatic  and  the  isothermal 
lines  we  are  leaving  but  a  narrow  margin  for  further  saving  ; 
and  if  that  saving  is  to  be  accomplished  by  complications 
of  mechanism,  by  increased  friction  and  clearance  losses, 
and  by  additional  cost  of  maintenance,  it  will  be  but  a 
doubtful  gain. 

The  device  of  cooling  the  air  by  the  injection  of  a  spray 
of  water  into  the  cylinder  is  probably  the  most  effective 
cooling  arrangement  that  has  ever  been  devised,  but  col- 
lateral objections  have  driven  it  completely  out  of  use,  in 
all  new  compressors  at  least,  in  the  United  States.  When 
the  spray  is  used  the  success  of  it  as  an  air-cooling  agent  is 
entirely  dependent  upon  the  mode  of  its  application.  The 
spray  can  only  possibly  effect  the  intended  purpose  when 
diffused  through  the  air  while  it  is  being  compressed,  or 
during  the  compression-stroke  of  the  piston.  It  can  only 
cool  the  air  while  it  is  hot,  or  while  it  is  being  heated  ;  so 
that  to  admit  the  water  with  the  incoming  air  is  only  to  let 
it  fall  inert  and  useless  to  the  bottom  of  the  cylinder,  to  be 
driven  out  by  the  piston.  Air  so  admitted  may  have  a 
quasi  usefulness  in  filling  the  clearance-space  at  the  end  of 
the  stroke,  but  it  can  do  little  or  nothing  toward  cooling 
the  air.  The  presence  of  the  water  may  also  make  it  un- 
safe to  run  the  compressor  at  a  speed  that  would  be  other- 
wise safe  and  proper.  With  the  use  of  water  in  the  com- 
pression-cylinder, whether  properly  injected  or  not,  no 
satisfactory  means  of  lubricating  the  surface  of  the  cylindei 
has  ever  been  found,  so  that  the  friction  of  the  piston  and 
the  loss  of  power  by  that  means  is  greater  than  with  other 
systems  of  compression.  The  piston  and  cylinder  surfaces 
also  wear  away  rapidly,  so  that  the  repair  cost  and  incon- 


68 


COMPRESSED   AIR. 


venience  is  greater  than  with  other  systems.  While  there 
is  no  compressor-builder,  that  I  know  of,  who  is  now  offer- 
ing a  compressor  furnished  with  injection-pumps,  there 


is  no  objection  to  any  builders  retaining  in  their  catalogues, 
as  they  do  at  this  writing,  the  standard  arguments  against 
the  injection  system,  because  it  helps  to  give  the  catalogue 


OF  COMPRESSION  IN  A    SINGLE   CYLINDER.     69 

a  formidable  appearance,  you  know,  and  no  one  is  harmed 
by  the  practice. 

The  size  of  the  compression-cylinder  is  a  thing  to  be 
thought  of  in  the  consideration  of  economical  air-compres- 
sion. Other  things  being  equal,  a  cylinder  of  small  diame- 
ter has  a  decided  advantage  over  a  large  one  in  cooling  the 
nir  during  compression.  In  a  large  cylinder  the  portion  of 
air  immediately  in  contact  with  or  lying  near  to  its  water- 
cooled  surfaces  will  be  cooled  by  the  contact,  but  the  air 
in  the  middle  of  the  cylinder  will  be  little  and  slowly 
affected.  A  number  of  small  compressors  will  show  better 
results,  as  regards  the  cooling  of  the  air,  than  a  large  com- 
pressor can  show.  This  has  something  to  do  with  an  indi- 
cator-diagram that  I  now  have  the  pleasure  of  offering 
(Fig.  7).  I  have  no  hesitation  in  saying  that  it  is  the  best 
and  most  satisfactory  diagram  made  by  a  single  compres- 
sion that  I  have  ever  seen.  The  scale  of  the  diagram  is 
30.  It  was  taken  from  one  of  a  series  of  small  compression- 
cylinders  entirely  submerged  in  water.  The  speed,  96  revo- 
lutions, was  not  slow,  so  that  the  result  was  remarkable. 
This  diagram  at  least  shows  conclusively  the  possibility  of 
compressing  in  a  single  cylinder  with  the  compression-line 
well  within  the  mean  of  theoretical  adiabatic  and  isothermal 
compression. 


CHAPTER  VIII. 

TWO-STAGE  AIR-COMPRESSION. 

WHAT  may  be  called  the  common  working-pressure  for 
compressed  air,  or  the  pressure  at  which  the  air  is  most  fre- 
quently used,  is  from  60  to  80  Ibs.  gauge,  or  s'ay  75  Ibs.,  or 
6  atmospheres.  This  is  the  usual  pressure  employed  in 
operating  rock  drills,  hoisting-engines,  pumps,  and  the  gen- 
eral line  of  mining,  tunnelling,  quarrying,  and  rock-excavat- 
ing machinery,  and  this  is  even  now  the  largest  general  field 
for  the  use  of  compressed  air.  While  most  of  the  com- 
pressed air  that  is  used  is  compressed  in  single  air-cylin- 
ders, usually  double-acting,  each  cylinderful  of  free  air 
being  compressed  and  delivered  by  each  single  stroke  of 
the  piston,  some  of  the  air  is  compressed  by  two-stage  com- 
pressors, or  by  compound  compression,  and  most  theorists 
advocate  the  two-stage  compression  system  for  ordinary 
pressures  ;  and,  as  a  matter  of  fact,  the  two-stage  compres- 
sors maintain  a  respectable  position  among  the  various 
competitors.  For  high  pressures  two-stage  or  triple  or 
even  quadruple  compression  may  be  necessary,  but  for  the 
pressures  that  are  commonly  employed,  at  least  up  to  6 
atmospheres,  the  ultimate  economy  of  two-stage  compres- 
sion is  still  an  open  and  debatable  question. 

When  we  come  to  look  into  two-stage  or  compound  com- 
pression, we  find  a  number  of  interesting  points  to  be  con- 

70  • 


TWO-STAGE  AIR-COMPRESSION.  7 1 

sidered,  and  the  air-compressing  problem  becomes  more 
complex.  The  conditions  in  detail  involved  in  the  opera- 
tion of  two-stage  compression  are  perhaps  better  exhibited 
where  the  cylinders  are  single-acting,  and  that  style  of 
compressor  we  will  first  consider.  I  offer  now — Figs.  8,  9, 
and  10— a  set  of  indicator-diagrams,  scale  80,  from  the  air- 


Fiff.S 


Fig.  10 


cylinders  of  a  two-stage  compressor.  The  cylinders  of  the 
compressor  from  which  these  cards  were  taken  were  each 
single-acting  arranged  tandem,  the  two  pistons  upon  the 
same  piston-rod,  and  doing  the  work  of  the  alternate  cylin- 
ders upon  the  alternate  strokes  of  the  engine,  the  steam- 
cylinder  also  being  in  line  with  the  air-cylinders  and  actu- 
ating the  same  piston-rod.  The  cylinders  were  20"  and 
i  if"  in  diameter  respectively,  and  the  stroke  18".  The 
capacity  ratio  of  the  two  cylinders,  deducting  the  area  of 
the  piston-rod  in  the  larger  cylinder,  was  i  :  .35.  Cards 
were  taken  from  both  air-cylinders  with  the  compressor  de- 


?2  COMPRESSED   AIR. 

livering  air  at  35  Ibs.,  at  40  Ibs.,  and  then  by  intervals  of 
10  Ibs.  all  the  way  up 'to  120  Ibs.  The  cards  here  pre- 
sented are  as  good  as  a  greater  number  for  bringing  out 
the  peculiarities  of  the  case.  Fig.  8  is  from  the  first  or  low- 
pressure  cylinder.  This  card  did  not  vary  in  any  particular 
throughout  the  whole  series  from  35  Ibs.  to  120  Ibs.,  and 
it  would  have  continued  the  same  no  matter  how  high  the 
terminal  or  delivery  pressure  of  the  second  cylinder  were 
carried.  A  tracing  was  made  of  one  of  these  cards  and 
laid  over  several  others  of  the  series,  and  the  variation  was 
so  slight  as  to  be  scarcely  discoverable  at  any  point. 

The  mean  effective  pressure  of  Fig.  8  is  15.8  Ibs.,  and 
the  terminal  pressure  is  35  Ibs.  While  the  terminal  press- 
ure in  this  first  cylinder  is  35  Ibs.,  it  does  not  mean  that 
if  the  two -stage  compressor  were  compressing  and  deliver- 
ing air  at  35  Ibs.  gauge,  the  first  cylinder  would  be  doing  all 
the  work  of  the  compressor.  It  is  to  be  remembered  that 
the  complete  work  of  air-compression  comprises  two  dis- 
tinct operations :  the  compression  of  the  air  to  the  re- 
quired pressure,  and  the  expulsion  or  delivery  of  the  air 
against  practically  the  same  pressure  in  the  air-pipes  or  in 
the  air-receiver.  In  the  case  that  we  are  now  considering, 
where  the  air  is  delivered  from  the  compressor  at  a  press- 
ure of  35  Ibs.,  the  first  cylinder  happens  to  do  all  of  the 
work  of  compression,  and  none  of  the  work  of  expulsion  or 
delivery.  In  any  case  of  two-stage  compression,  if  either 
cylinder  is  to  be  called  distinctively  the  "  compressing"  cyl- 
inder, that  term  always  belongs  to  the  first  cylinder  rather 
than  to  the  second.  If  our  two-stage  compressor  were  de- 
livering air  at  a  pressure  higher  than  35  Ibs.,  the  first  cylin- 
der would  still  compress  the  air  to  35  Ibs.  as  before,  or 
would  do  only  a  portion  of  the  total  compression,  and  of 


TWO-STAGE  AIR-COMPRESSION.  73 

course  none  of  the  delivery.  The  height  to  which  the  first 
cylinder  will  continually  compress  the  air  is  determined 
by  the  relative  capacities  of  the  two  cylinders  modified  to 
some  extent  by  the  cooling  of  the  air  that  may  be  effected 
in  its  passage  from  one  cylinder  to  the  other.  The  work 
of  the  second  cylinder  when  the  compressor  is  delivering 
the  air  at  35  Ibs.  is  shown  by  Fig.  9,  taken  from  that  cylin- 
der. The  delivery-line  ba  in  this  case  would  be  a  per- 
fectly horizontal  line  if  the  movement  of  the  piston  were 
uniform  throughout  the  stroke,  the  rise  and  fall  of  the  line 
corresponding  approximately  to  the  acceleration  and  re- 
tardation of  the  piston. 

At  whatever  pressure  the  compressed  air  may  be  deliv- 
ered by  the  compressor  the  mean  effective  pressures  for 
the  two  distinct  operations  of  compression  are  never  alike. 
The  mean  effective  pressure  for  compression  only  is  al- 
ways lower  than  the  M.E.P.  for  delivery  only,  and  of 
course  also  lower  than  for  the  combined  operation  of  com- 
pression and  delivery  as  performed  in  a  single  cylinder. 
In  the  compression  table  II.  columns  6  and  7  give  the 
mean  effective  pressures  for  the  whole  stroke  when  all  of 
the  work  of  compression  and  delivery  is  done  in  a  single 
cylinder,  column  6  being  for  isothermal  and  column  7  being 
for  adiabatic  compression.  In  the  same  table  columns  8 
find  9  give  respectively  the  isothermal  and  the  adiabatic 
M.E.P.  for  the  compression  part  only  of  the  stroke  of  a 
single  air- cylinder. 

Resuming  now  our  compound  compression,  and  referring 
Again  to  Fig.  8,  we  notice  that  its  mean  effective  pressure — 
15.8 — is  greater  than  the  pressures  given  in  either  columns 
B  or  9  for  compression  only  to  35  Ibs.,  where  the  entire 
work  of  the  compressor  is  done  in  a  single  air-cylinder. 


74  COMPRESSED    AIR. 

The  table  referred  to,  as  we  have  previously  stated,  has 
nothing  to  do  with  compound  compression,  but  the  com- 
parison of  figures  might  provoke  a  suspicion  that  in  com' 
pound  or  two-stage  compression  we  are  doing  the  same 
work  of  compression  as  in  the  single  air-cylinder,  but  at 
greater  expense,  and  it  is  therefore  proper  to  refer  to  it 
here.  The  case  represented  is  different  in  more  than  one 
particular.  In  single-stage  compression  the  compression  is 
all  done  in  the  one  cylinder,  and  throughout  the  entire 
compression-stroke  the  same  quantity  or  weight  of  air  is 
acted  upon.  In  Fig.  8  we  are  not  doing  the  entire  com- 
pression part  of  the  work  in  the  one  cylinder,  although  it  is 
begun  there,  and  the  weight  of  air  acted  upon  is  not  the 
same  throughout  the  stroke.  While  at  the  beginning  of  the 
stroke  the  air  acted  upon  is  the  free  air  contained  in  the 
first  cylinder  and  just  admitted  from  the  atmosphere,  this 
continues  only  for  the  first  half  of  the  stroke,  and  for  the 
latter  part  of  the  stroke  the  whole  body  of  air  then  undergo- 
ing compression  consists  not  only  of  all  the  contents  of  the 
first  cylinder  that  have  not  been  expelled  by  the  advancing 
piston,  but  also  of  the  entire  contents  of  the  passage  con- 
necting the  two  cylinders,  and  the  contents  of  that  part  of 
the  second  cylinder  which  has  been  vacated  by-its  retreat- 
ing piston.  Fig.  8  shows  the  compression  beginning  at  a, 
at  the  beginning  of  the  stroke,  and  with  the  free  air  con- 
tents of  the  first  cylinder  alone.  This  goes  on  until  the 
point  o  is  reached,  near  the  middle  of  the  stroke,  and  then 
communication  is  opened  with  the  air-passage  that  connects 
the  cylinders,  and  through  that  with  the  second  cylinder. 
When  the  previous  compression-stroke  of  the  first  cylinder 
ended,  the  passage  connecting  the  cylinders  was  filled  with 
air  compressed  to  35  Ibs.,  and  by  the  action  of  the  valves 


TWO-STAGE  AIR-COMPRESSION.  75 

this  passage  was  then  for  a  time  shut  off  from  communica- 
tion with  either  cylinder.  This  passage,  in  fact,  remains 
shut  off  from  communication  with  either  cylinder  during 
the  whole  of  the  return  stroke,  while  the  first  cylinder  is 
being  filled  with  a  fresh  charge  of  free  air,  and  while  the 
compressed  air  in  the  smaller  cylinder  is  being  expelled 
into  the  discharge-pipe  and  the  air-receiver.  When  the 
return  or  intake  stroke  of  the  larger  cylinder  has  ended, 
which  return  stroke  is  the  delivery-stroke  of  the  smaller 
cylinder,  and  when  the  compressed  air  has  all  been  expelled 
from  the  smaller  cylinder  by  its  piston  reaching  the  end  of 
it,  then  the  return  stroke  of  the  smaller  cylinder  commences, 
this  stroke  being  of  course  coincident  with  the  next  com- 
pression-stroke of  the  larger  cylinder.  With  the  com- 
mencement of  the  return  stroke  of  the  smaller  piston  the 
air  confined  in  the  connecting  passage  begins  to  re-ex- 
pand and  to  flow  into  the  smaller  cylinder.  The  pressure 
is  thus  falling  in  the  air-passage,  on  account  of  its  supply- 
ing the  smaller  cylinder,  and  at  the  same  time  compression 
is  going  on  in  the  larger  cylinder,  and  the  pressure  in  it  is 
rising.  These  simultaneous  operations  go  on  until  at 
length  the  point  o  is  reached,  where  the  pressure  in  the 
larger  cylinder  exceeds  the  pressure  in  the  air-passage  and 
in  the  smaller  cylinder,  and  the  air  from  the  larger  cyl- 
inder begins  to  flow  into  the  air-passage,  and  at  the  same 
time  the  entire  contents  of  the  air-passage  and  of  the 
smaller  cylinder  become  constituent  parts  of  the  body  of 
air  that  is  being  compressed  by  the  advancing  piston  of 
the  larger  cylinder,  and  thereafter  until  the  end  of  the 
stroke  the  compression  of  the  combined  contents  of  large 
cylinder,  air-passage,  and  small  cylinder  goes  on  together. 
The  last  one  third  of  the  compression-stroke  in  Fig.  8  and 


76  COMPRESSED   AIR. 

ub  in  Fig.  9  or  10  represent  the  same  operation  of  com- 
pression, the  line  in  Fig.  8  showing  a  somewhat  higher 
pressure  than  in  Fig.  9  or  10  on  account  of  the  friction  to 
be  overcome  in  passing  the  valves  and  passages. 

The  mean  effective  pressure  for  the  combined  operation 
of  compressing  and  expelling  the  air  at  35  Ibs.,  or  for  the 
whole  operation  of  air-compression  so  termed,  when  per- 
formed adiabatically  in  a  single  cylinder  is,  theoretically, 
21.6  Ibs.  Practically,  without  any  special  arrangements  for 
cooling  the  air,  the  M.E.P.  usually  falls  somewhat  below  the 
above  figure,  as  the  air  inevitably  loses  more  or  less  of  its 
heat  during  the  operation.  If  we  consider  Fig.  8  in  con- 
nection with  Fig.  9,  they  together  represent  the  whole  op- 
eration of  compression  to  35  Ibs.  by  two-stage  compression, 
Fig.  8  representing  the  compression  of  the  air  and  Fig.  9 
representing  its  expulsion  or  delivery.  The  mean  effective 
pressure  of  Fig.  8  is,  as  we  have  seen,  15.8,  and  that  of  Fig. 
9  is  16.4  Ibs.  But  it  must  be  remembered  that  the  diameters 
of  the  two  cylinders  are  quite  different,  and  16.4  Ibs.  in  the 
nj"  cylinder  is  only  equal  in  power  to  5.65  Ibs.  in  the  20" 
cylinder,  and  15.8  +  5.65  =  21.45  Ibs,  a  mean  effective  pres- 
sure quite  close  to  what  might  have  been  expected  for  the 
entire  operation  of  compressing  air  to  35  Ibs.  without  any 
device  for  cooling  the  air.  When  we  remember  that  the  use 
of  two  cylinders  instead  of  one  for  the  same  operation  of  com- 
pression means  necessarily  a  greater  first  cost  for  the  appar- 
atus, to  the  builder  if  not  to  the  purchaser,  a  larger  number 
of  parts,  increasing  the  liability  to  accidents  and  delays,  and  a 
greater  amount  of  friction,  both  in  the  air  and  in  the  machine, 
to  be  constantly  overcome,  it  is  evident  that  two-stage  com- 
pression of  itself  costs  more  than  single-stage  compression. 

While  these  diagrams  were  being  taken  the  compressor 


TWO-STAGE  AIR-COMPRESSION.  77 

was  run  at  about  80  revolutions  per  min.,  or  240  feet  of 
piston  travel  per  min.,  throughout.  At  this  speed  the  indi- 
cated horse-power  of  Fig.  8  for  the  first  cylinder  is  18.05. 
and  that  of  Fig.  9  from  the  second  cylinder  is  6.46,  their 
sum  being  24.51.  Fig.  10  is  from  the  smaller  cylinder  when 
compressing  to  70  Ibs.  The  M.E.P.  of  Fig.  10  being  43  4, 
and  the  indicated  horse-power  being  17.1,  the  I.H.-P.  for 
Fig.  8  being,  as  before,  18.05,  their  sum  is  35.15.  When 
compressing  and  delivering  air  at  70  Ibs.,  as  indicated  by 
Figs.  8  and  10,  it  will  be  noticed  that  the  I.H.-P.  of  the 
two  cylinders  is  nearly  equal,  and  it  would  thus  seem  that 
the  ratio  of  the  cylinder  capacities  to  each  other  was  ap- 
proximately correct  for  that  pressure.  The  relative  diame- 
ters and  areas  of  the  two  cylinders  may  have  been  deter- 
mined upon  this  assumption.  An  incomplete  theory  is 
more  easily  satisfied  than  one  which  takes  cognizance  of  all 
the  conditions. 

The  arrangement  of  the  tandem,  single-acting,  two-stage 
compressing  cylinders  is  about  as  bad  a  one  as  could  be 
devised  for  an  air-compressor,  and  no  possible  change  in 
the  relative  capacities  of  the  two  cylinders  can  make  it 
right.  The  trouble  in  the  case  is  that  while  the  sum  of  the 
indicated  horse-powers  as  computed  from  the  actual  en- 
closed areas  of  the  two  cards  is  correct  as  representing  the 
total  horse-power  consumed  in  the  operation,  it  does  not 
correctly  represent  the  actual  distribution  of  the  resistances 
as  encountered  in  the  opposite  strokes  of  the  engine.  The 
back  piessure  in  the  second  cylinder,  which  thus  far  has 
not  been  thought  of,  imperatively  demands  recognition  and 
accounting  with  as  modifying  the  total  resistances  encoun- 
tered. The  back-pressure  line,  or,  perhaps  more  correctly, 
the  return-pressure  line,  cxub,  as  we  have  seen,  starting  at 


78  COMPRESSED   AIR, 

c,  represents  for  nearly  one-half  the  stroke  the  re-expansion 
of  the  contents  of  the  air-passage.  This  re-expansion  goes 
on  in  the  passage  and  in  the  smaller  cylinder  combined  until 
the  point  x  is  reached,  when  the  compression  going  on  in  the 
larger  cylinder  has  brought  its  contents  up  to  the  same  pres- 
sure. Then  after  a  short  interval,  xu,  occupied  in  securing  a 
sufficient  excess  of  pressure,  and  in  reversing  the  movement 
from  expansion  to  compression,  the  compression  continues 
from  u  to  the  end  of  the  stroke,  when  the  pressure  of  35  Ibs. 
is  again  reached.  As  the  whole  of  Fig.  8  is  always  the  same, 
no  matter  what  may  be  the  working  pressure  of  the  compres- 
sor, so  that  it  is  not  below  35  Ibs.,  so  also  the  return  line  of 
the  diagram  from  the  second  cylinder  is  always  the  same,  and 
the  only  change  in  the  pair  of  Figs.  8  and  9  or  8  and  10  for 
different  delivery-pressures  is  in  the  upper  line  ba,  the  com- 
pression- and  delivery-line  of  the  second  cylinder.  When 
compressing  to  35  Ibs.  only  there  is  no  compression  in  the 
second  cylinder,  and  its  whole  stroke  ic  occupied  in  delivery. 
At  the  beginning  of  the  stroke  the  resistance  against  the  high- 
pressure  piston  is  represented  by  the  height  of  the  vertical 
line  bd.  The  resistance  at  any  point  of  the  stroke  would 
be  represented  by  a  vertical  line  at  that  point  drawn  from 
the  line  ba  down  to  the  atmosphere-line,  and  the  total 
resistance  for  the  working-stroke  is  represented  by  the 
enclosed  area,  bdea.  This  means  that  the  total  back 
pressure,  bdec,  is  to  be  added  to,  or,  rather,  is  not  to  be 
deducted  from,  the  work  of  the  compression  and  delivery- 
stroke  of  the  high-pressure  cylinder.  During  this  working- 
stroke  of  the  high-pressure  cylinder  the  low-pressure  piston 
is  making  its  return  stroke  and  allowing  its  cylinder  to  refill 
with  air  at  atmospheric  pressure.  The  pressure  upon  each 
side  of  the  low-pressure  piston  upon  its  return  stroke  is 


TWO-STAGE  AIR-COMPRESSION.  79 

practically  that  of  the  atmosphere,  and  therefore  no  resist- 
ance of  any  magnitude  is  to  be  taken  into  account  as  in- 
creasing or  diminishing  the  total  work  of  the  high-pressure 
cylinder  for  its  delivery-stroke.  When,  however,  the  low- 
pressure  cylinder  is  doing  its  work  of  compression,  it  is 
assisted  in  its  work  by  the  return  or  back  pressure  of  the 
high-pressure  cylinder,  which  acts  upon  the  high-pressure 
piston  in  the  same  linear  direction  as  the  low-pressure 
piston  is  travelling.  The  back  pressure,  bdec,  which  is 
added  to  the  work  of  the  high-pressure  cylinder  for  its  de- 
livery-stroke, as  represented  by  the  enclosed  area  bac,  is  to 
be  deducted  from  the  work  of  the  low-pressure  cylinder  for 
its  compression-stroke  as  represented  by  Fig.  8. 

If  now  we  go  over  the  series  of  indicator-cards,  comput- 
ing the  indicated  horse-power  of  each,  adding  the  I.H.-P.  of 
the  back  pressure  to  the  I.H.-P.  of  each  of  the  high-pressure 
cards,  and  deducting  the  same  from  the  I.H.-P.  of  the  low- 
pressure  card,  as  above  described,  we  find  that  the  net  re- 
sistance for  the  alternate  strokes  is  very  inequitably  dis- 
tributed. The  figures  for  compressing  to  120  Ibs.  are  also 
given  to  aid  the  comparison,  although  the  delivery  or  high- 
pressure  card  for  that  pressure  is  not  shown.  The  case  will 
stand  like  this: 

M.E.P.  of  low-pressure  cylinder  15.8  Ibs.,  I.H.-P.  18.05. 
M.E.P.  of  return  stroke  of  high  pressure  cylinder  20.1,  I.H.-P.  7.8$. 
Then   18.05  —  7.88=10.17,  the  constant  net  I.H.-P.  for  the 
compression-stroke' of   the   low-pressure   cylinder    or   the 
return  stroke  of  the  high-pressure  cylinder. 

M.E.P.  of  high-pressure  cyl.  at    35  Ibs.  16.4,  I.H.-P.    6.46. 
"     "  "          "    "    70   "     43.4         "         17.1 

"     "          "          "     "  120   "    65.7       "         25.89. 

Then  adding  to  these  results   the  I.H.-P.  for  the  return 


8O  COMPRESSED   AIR. 

stroke,  which   should  not   have  been   deducted  from    the 
delivery-stroke,  we  have: 

6.46+  7.88  =  14.34  when  delivering  at    35  Ibs. 
17.1    +7.88  =  24.98       "  "     70    " 

25.89  +  7-88-33.77       "  "          "   120    " 

As  these  several  results  for  the  delivery-stroke  are  sue 
cessively  to  be  compared  with  the  constant  I.H.-P.  10.17 
for  the  initial  compression-stroke,  it  will  be  seen  that  even 
when  delivering  the  air  at  but  35  Ibs.  the  delivery-stroke  of 
the  high-pressure  cylinder  takes  nearly  i|  times  the  power 
required  for  the  return  stroke.  When  compressing  to  70 
Ibs.  under  the  above  arrangement  the  delivery-stroke  takes 
nearly  2^  times  the  power  of  the  return  stroke,  and  when 
compressing  to  120  Ibs.  it  takes  more  than  3  times  as  much. 

The  total  power  required  for  the  above  compressor  at 
the  speed  given  is  : 

35  Ibs.  —  10.17  +  14-34  =  24-51 

70    "    —10.17  +  24.98  =  35.15 

120    "    -10.17  +  33.77  =  43.94 

The  volume  of  free  air  compressed  and  delivered  at  either 
pressure  is  262  cu.  ft.  per  min. 

The  loss  by  friction  in  a  two-stage  compressor  should  be 
greater  than  in  a  single-stage  compressor  of  the  same  free 
air  capacity  and  working  to  the  same  pressure,  and  the 
total  friction  of  single-acting  cylinders  must  be  propor- 
tionately greater  than  that  of  double-acting  cylinders, 
so  that  if  for  a  common  single-stage  double-acting  com- 
pressor we  allow  10  per  cent  for  the  total  friction  of  the 
machine,  it  is  probable  that  15  per  cent  is  not  too  great 
to  allow  for  the  arrangement  that  we  have  been  considering 
above. 


CHAPTER   IX. 

TWO-STAGE    COMPRESSION,    SINGLE-ACTING    TANDEM, 
DOUBLE-ACTING   TANDEM,    AND   CROSS-COMPOUND. 

I  REFER  again  in  this  chapter  to  indicator-cards  Figs.  8 
and  10  from  the  single-acting,  two-stage,  tandem  air-cylin- 
ders delivering  the  air  at  70  Ibs.  I  reproduce  these  cards 
with  some  combinations  resulting  from  them  to  show 
graphically  how  the  net  resistances  are  distributed  through- 
out the  alternate  strokes. 

When  the  compressor  is  in  operation,  both  pistons  are 
always  exposed  to  the  atmospheric  pressure  upon  the  sides 
nearest  to  each  other.  The  other  side,  or  the  compressing 
side,  of  the  larger  piston  is  also  exposed  to  the  atmospheric 
pressure,  or  very  nearly  so,  during  its  intake  stroke.  The 
compressing  side  of  the  smaller  piston  is  never  exposed  to 
the  atmospheric  pressure  when  the  compressor  is  in  opera- 
tion. During  the  intake  stroke  of  the  smaller  cylinder, 
while  it  is  receiving  the  air  that  is  being  compressed  in  the 
larger  cylinder,  its  piston  is  subject  to  the  pressure  that  is 
due  to  that  initial  compression.  As  both  of  the  pistons 
are  upon  one  rod,  whatever  pressure  there  may  be  upon  the 
smaller  piston  when  the  larger  piston  is  doing  its  work  is 
just  so  much  help  for  the  larger  piston,  and  consequently 
cbde  of  Fig.  10  is  to  be  deducted  from  the  total  work  of 
Fig.  8.  In  Fig.  1 1  the  area  cbde,  representing  this  reacting 
pressure,  has  been  reduced  to  the  scale  corresponding  to 
the  relative  area  of  the  larger  cylinder,  and  has  been  super- 
Si 


82 


COMPRESSED   AIR. 


imposed  upon  Fig.  8.     It  will  be  seen  that  until  the  point  /' 
is  reached  the   steam-cylinder,  or  whatever  motor   is  em- 


Fig.  11 

ployed,  has  "  less  than  nothing "  to  do,  and  if  the  com- 
pressor were  running  slowly,  it  would  be  apt  to  give  a  per- 
ceptible jump  ahead  just  after  passing  this  centre.  This 
has  been  actually  observed  to  occur  in  a  compressor  of  this 
type.  In  Fig.  12  the  two  diagrams  have  been  combined 


U^-^  Fig.  12 

into  a  single  figure,  with  AB  as  the  line  of  no  resistance. 
This,  it  will  be  remembered,  represents  the  distribution  of 
the  resistance  for  the  compression-stroke  of  the  larger  pis- 
ton. For  nearly  one  quarter  of  the  stroke,  considering  here 
the  air-cylinders  only,  and  with  no  reference  to  the  driving 
power  of  the  steam-cylinders,  the  larger  piston  has  a  force 
behind  it  greater  than  the  resistance  in  front  of  it. 
From  the  point  i  the  net  resistance  begins  to  rise  before 
the  larger  piston,  and  continues  to  rise  until  the  ex- 
treme end  of  the  stroke,  except  for  a  slight  interval  at 
the  middle.  Fig.  13  represents  the  resistance  for  the  return 


Fig.  13 

stroke,  which  is  the  delivery-stroke  of  the  smaller  piston. 
This  diagram  is  the  same  as  baed  oi  Fig.  10,  but  drawn  to 
the  scale  of  the  larger  cylinder  for  comparison.  It  has 


TWO-STAGE    COMPRESSION,  ETC.  83 

also  for  convenience  been  reversed.  It  is  easy  enough 
by  a  glance  at  Figs.  12  and  13  to  see  the  difference  in  the 
resistances  for  the  alternate  strokes.  If  the  compressor 
were  delivering  the  air  at  35  Ibs.,  instead  of  at  70  Ibs., 
the  upper  line  of  Fig.  13  would  approximately  follow  the 
dotted  line  ba,  and  the  resistance  would  be  practically  uni- 
form for  the  entire  stroke.  Fig.  12,  representing  the  alter- 
nate stroke,  would  remain  precisely  the  same  whether  the 
smaller  cylinder  were  delivering  the  air  at  35  Ibs.,  at  70  Ibs., 
or  at  any  higher  pressure,  and  even  at  the  lower  pressure 
the  resistance  for  this  stroke  would  not  be  as  great  as  for 
the  delivery-stroke. 

It  is  evident  that  the  resistance  for  the  alternate  strokes 
could  not  be  equalized  by  changing  the  relative  capacities 
of  the  two  cylinders.  To  decrease  the  smaller  cylinder 
would  indeed  tend  toward  an  equalization  of  the  resistances 
by  allowing  the  first  cylinder  to  do  more  work  and  com- 
press the  air  to  a  higher  pressure  ;  but  to  raise  the  pressure 
in  the  first  cylinder  would  be  to  defeat  the  purpose  for 
which  the  two-stage  compression  is  adopted — that  of  allow- 
ing a  cooling  of  the  air  and  a  reduction  of  its  volume  before 
its  compression  is  too  far  advanced. 

As  Figs.  12  and  13  represent  the  resistances  for  the  alter- 
nate strokes  of  single-acting  cylinders,  these  resistances  may 
be  added  together  and  we  may  combine  them,  as  is  done 


Fig.  14 

in  Fig.  14,  and  we  then  have  the  diagram  for  either  stroke 
of  tandem  double-acting  cylinders  of  the  same  sizes.     This 


84  COMPRESSED    AIR. 

of  course  represents  double  the  free  air  capacity  of  the 
single-acting  cylinders.  Fig.  15  is  a  theoretical  diagram  of 
a  double-acting  single-stage  compression  cylinder  of  the 
same  capacity,  the  assumed  compression-line  being  the 
mean  of  the  adiabatic  and  the  isothermal  curves.  The 
maximum  resistance  for  the  stroke  in  the  two-stage  double- 
acting  compressor  is  only  three  fourths  of  the  maximum 
resistance  for  the  single-stage  compressor.  The  resistance 
at  the  beginning  of  the  stroke  is  not  as  low  in  the  former  as 


fig.  15 

in  the  latter,  and  the  distribution  of  the  resistance  over  the 
whole  stroke  is  decidedly  more  uniform.  As  to  the  total 
effective  resistance  for  the  stroke,  as  we  have  here  devel- 
oped it,  the  two-stage,  compressor  shows  no  advantage  over 
the  single-stage  even  while  ignoring  the  additional  friction 
of  the  former.  In  fact,  the  mean  effective  resistance  of 
Fig.  15  is  somewhat  less  than  that  of  Fig.  14.  "This  might 
have  been  expected,  because  in  the  cylinders  from  which 
Fig.  14  was  evolved  the  full  benefits  of  water-jacketing  were 
not  employed,  the  cylinder-heads,  for  instance,  not  being 
jacketed. 

We  know  tolerably  well  the  importance  of  employing  all 
available  means  (if  they  don't  cost  too  much)  of  cooling  the 
air  while  it  is  undergoing  compression;  and  as  the  two-stage 
method  of  compression  is  only  adopted  for  the  sake  of  the 
cooling  that  may  be  effected  between  the  stages,  it  may  be 


TWO-STAGE    COMPRESSION,  ETC.  8$ 

well  right  hereto  look  a  little  into  the  operation  of  a  cooler, 
or,  as  it  is  commonly  called,  an  *'  intercooler,"  placed  be- 
tween the  cylinders  of  a  tandem  two-stage  air-compressor. 
It  is  assumed  and  asserted  that  by  the  use  of  the  inter- 
cooler  a  complete  cooling  of  the  air,  and  of  all  the  air, 
compressed  by  the  first  cylinder  is  effected  before  it  is  sub- 
jected to  the  second  and  final  compression  and  delivery. 
Indicator-cards  would  show  conclusively,  by  the  relative 
volume  delivered  to  the  second  cylinder,  the  actual  cooling 
that  was  accomplished.  I  regret  that  I  am  not  now  able  to 
present  indicator-cards  from  a  compressor  of  this  type.  I 
cannot  learn  that  any  actual  cards  from  an  American 
double-acting,  tandem,  two-stage  compressor  with  an  inter- 
cooler  have  ever  been  published. 

At  the  beginning  of  the  operation  of  compression  in 
a  compressor  of  this  type,  remembering,  as  I  have  remarked 
before,  that  the  function  of  the  first  cylinder  is  entirely  one 
of  compression,  and  that,  if  either  cylinder  is  to  be  called 
distinctively  the  "  compressing  "  cylinder,  it  should  be  the 
first  one  rather  than  the  second,  the  body  of  air  to  be  acted 
upon  by  the  first  piston  consists,  at  the  beginning  of  any 
stroke,  of  the  entire  contents  of  the  first  cylinder  and  also 
of  the  air  contained  in  the  intercooler  at  the  time  and  in 
the  passages  connecting  the  intercooler  with  each  cylinder. 
As  the  compressing  piston  advances  in  the  first  cylinder  the 
total  compression-chamber  at  any  time  after  the  beginning 
of  the  stroke  consists  at  that  time  of  the  remaining  portion 
of  the  first  cylinder  still  untraversed  by  its  piston,  of  the 
intercooler  and  its  connecting  passages  as  before,  and 
of  that  portion  of  the  second  cylinder  that  has  been 
vacated  by  its  retreating  piston.  The  actual  situation  is 
not  quite  as  simple  as  our  statement  of  it  here,  as  we  can 


86  COMPRESSED    AIR. 

see  a  little  later.  In  standard  compressors  of  the  type  that 
we  are  considering  the  piston  areas  and  consequently  the 
cubical  capacities  of  the  cylinders  usually  bear  to  each  other 
about  the  ratio  of  10  :  4.  Now  representing  the  capacity 
of  the  first  cylinder  by  10,  that  of  the  passage  connecting 
it  with  the  cooler  by  2,  of  the  cooler  itself  by  2,  of  the 
passage  to  the  second  cylinder  by  2,  and  the  total  capacity 
of  the  second  cylinder  by  4,  we  may  be  able  to  see  what  the 
intercooler  has  to  operate  upon  at  any  given  time,  and  what 
chance  it  has  to  completely  cool  all  the  air.  I  assume,  of 
course,  that  the  cooler  does  thoroughly  cool  all  the  air  that 
passes  through  it,  and  at  the  pressure  at  which  it  passes 
through.  I  see  no  reason  why  it  should  not  be  made  effi- 
cient in  this  respect. 

The  operation  at  successive  stages  of  the  compression- 
stroke  is  as  follows  :  At  the  beginning  of  the  stroke  of  the 
first  cylinder  the  entire  body  of  air  to  be  compressed  is 
represented  by  16,  comprised  like  this  :  The  contents  of 
the  first  cylinder  10,  passage  to  cooler  2,  contents  of  cooler 
2,  passage  to  second  cylinder  2.  Of  this  volume  of  air  only 
the  first  10  parts,  the  contents  of  the  first  cylinder,  is  "free 
air  "  The  remainder,  the  contents  of  the  cooler  and  the 
connecting  passages,  having  been  compressed  upon  the 
previous  stroke  to  the  pressure  at  which  the  air  is  finally 
delivered  to  the  second  cylinder,  and  at  the  end  of  the 
stroke  having  been  shut  off  by  itself  apart  from  either  cylin- 
der, stands  now  at  a  pressure  somewhat  above  35  Ibs.  gauge. 
As  the  stroke  goes  on,  and  the  piston  of  the  second  cylin- 
der recedes,  this  air  in  the  cooler,  and  passages  begins  to 
re-expand,  and  to  flow  into  the  second  cylinder,  and  the 
pressure  of  this  air  consequently  falls.  At  the  same  time 
compression  is  going  on  without  cooling  in  the  first  cylin- 


TWO-S7^AGE    COMPRESSION,  ETC.  87 

der.  The  total  free  air  contents  of  the  first  cylinder  are 
compressed  independently  until  the  middle  of  the  stroke  is 
reached,  or  a  little  beyond  that,  and  a  pressure  of  about  20 
Ibs.  is  attained  in  the  cylinder  without  any  of  the  cooling 
and  power-saving  effects  of  the  intercooler  being  felt  upon  it. 
Practically  none  of  the  air  of  any  compression-stroke  flows 
through  the  intercooler  until  after  the  middle  of  that  stroke 
is  reached.  Assuming  that  the  pressures  in  the  compress- 
ing cylinder  and  in  the  cooler  and  passages  have  become 
equal  when  the  middle  of  the  stroke  is  reached,  and  that  at 
that  point  the  piston  of  the  first  cylinder  begins  to  act  upon 
the  whole  body  of  air  at  once,  the  air  then  under  compres- 
sion will  be  :  Contents  of  first  cylinder  5,  of  passage  to 
cooler  2,  of  cooler  2,  of  passage  to  second  cylinder  2,  and 
contents  of  second  cylinder  2 — total  13  ;  and  ^  of  this  — 
.307  has  already  passed  the  cooler  and  can  be  no  more 
cooled  by  it,  and  T\  =  .538  has  not  yet  reached  the  cooler, 
and  has  been  compressed  thus  far  without  any  cooling 
effect  whatever  from  it.  At  three  quarter  stroke  the  body 
of  air  under  compression  will  be  distributed  as  follows  : 
Remaining  contents  of  first  cylinder  2.5,  passage  to  cooler  2, 
cooler  2,  passage  to  second  cylinder  2,  and  contents  of  sec- 
ond cylinder  3 — total  11.5;  and  of  this  body  5/11.5  =  .434 
has  already  passed  the  cooler  and  cannot  be  further  af- 
fected by  it,  and  4.5/11.5  =  .39  has  not  yet  reached  the 
cooler,  and  has  not  been  cooled  at  all  by  it.  When  the  end 
of  the  stroke  is  reached,  the  air  is  distributed  like  this  : 
First  cylinder  o,  passage  2,  cooler  2,  passage  2,  second  cylin- 
der 4 — total  10  ;  and  of  this  -f$  =  .2  has  not  yet  reached 
the  cooler  and  has  undergone  the  whole  compression  from 
atmospheric  pressure  without  cooling,  and  all  of  the  con- 


COMPRESSED    AIR. 

tents  of  the  second  cylinder  have  been  compressed  and 
heated  more  or  less  after  passing  the  cooler. 

The  intercooler  applied  in  this  way  would  seem  to  be  a 
rather  crude  and  not  very  efficient  device  and  when  con- 
fidence in  the  virtues  of  the  intercooler  leads  to  the  discard 
ing  of  the  most  valuable  feature  of  water-jacketing, — the 
j  icketing  of  the  cylinder-heads, — and  when,  for  the  same 
work  of  compression,  two  cylinders  are  employed  instead  of 
one,  with  the  consequent  increase  of  friction  in  the  ma- 
chine, and  with  the  increased  friction  also  of  the  air  past  a 
double  set  of  valves  and  through  longer  and  more  tortuous 
passages,  it  would  surely  seem  to  require  a  voluminous 
argument  to  show  in  the  system  any  superiority  over  the 
single-cylinder  completely  water-jacketed  compressor  for 
the  commonly  employed  working  pressures. 

Figs.  16  and  17  are  indicator-cards  from  two-stage  air- 
cylinders  operated  by  cross-connected  Corliss  engines  with 


Fiff.  16 

the  cranks  at  right  angles.  The  piston  rod  of  each  steam- 
cylinder  in  this  style  of  compressor  is  carried  back  through 
the  head  and  into  the  air-cylinder,  the  low-pressure,  or 
intake,  air-cylinder  being  placed  tandem  to  one  steam-cylin- 
der, and  the  high- pressure,  or  delivery,  air-cylinder  being 
connected  in  the  same  way  to  the  other  steam-cylinder. 


TWO-STAGE    COMPRESSION,  ETC.  89 

These  cards  are  reproduced  here  to  show  the  characteristics 
of  this  style  of  compressor  as  compared  with  the  tandem 
air-cylinder  arrangement.  They  may  to  the  general  reader 
possess  an  additional  interest  from  the  fact  that  the  original 
cards  were  taken  in  South  Africa,  where  there  are  now  in- 
stalled a  large  number  of  high-duty  air-compressors  of 
American  manufacture.  As  the  cards  have  been  twice 


Fly.  17 

retraced,  they  should  not  be  too  closely  scrutinized.  The 
intake  cylinder  was  31"  dia.  X  42"stroke,  and  the  delivery- 
cylinder  19.$"  dia.  X  42"  stroke.  The  cards  were  taken 
with  the  compressor  running  at  40  revolutions  per  minute. 
The  scale  of  the  first  card  is  20,  and  that  of  the  second 
card  is  60. 


CHAPTER  X. 
THE  POWER  COST  OF  COMPRESSED  AIR. 

WHAT  is  the  actual  power  cost  of  a  cubic  foot  of  com- 
pressed air  at  any  given  pressure  ?  This  is  only  one  end  of 
the  question  of  economy  in  employing  compressed  air  for 
power  transmission,  and  besides  the  ends  of  it  there  is  a 
middle  of  some  magnitude.  The  question  of  practical 
economy  has  many  complications,  and  whether  air  shall  be 
employed  in  a  given  case  may  be  determined  by  considera- 
tions far  removed  from  those  that  we  would  recognize  as 
bearing  upon  the  economy  of  it.  There  are  many  cases 
where  at  the  present  time  the  use  of  compressed  air  is  im- 
perative, whatever  its  cost ;  but  still  as  the  bill  has  to  be  paid 
it  is  well  to  compute  it.  In  considering  the  actual  cost  of 
compression  we  will  not  now  look  into  all  the  possible 
economies  of  the  case,  but  will  try  to  get  at  the  actual  cost 
according  to  the  common  practice  of  air-compression  at 
the  present  time. 

Say,  then,  that  we  have  a  steam-actuated  air-compressor, 
with  steam-  and  air-cylinders  both  20"  dia.  X  24"  stroke,  at 
75  revolutions  per  min.,  using  steam  at  80  Ibs.  and  com- 
pressing air  to  80  Ibs.  The  case  will  then  be  like  this  : 

Power  required  by  air-cylinder  : 

2o2  X  .7854  X  36.6  X  300  -4-  33,000  =  104.53  H.-P. 
IO4-53  +  I0  Per  cent.  =  114.98  H.-P. 

90 


THE  POWER    COST  OF  COMPRESSED   AIR.       9 1 
Volume  of  free  air  compressed  by  air-cylinder  : 

2o2  X  .7854  X  300  -f-  144  =  654.5. 

654.5  —  10  per  cent  =  589  cu.  ft.  free  air. 
589  X  .1552  =  91.4  cu.  ft.  at  80  Ibs. 

Power  of  steam-cylinder  (steam  80  Ibs.,  cut-off  .25,  M.E.P. 
40.29): 

2o2  X  .7854  X  40.29  X  300  -f-  33,000  =  115.06  H.-P. 
Volume  of  steam  used  : 

20"  X  .7854  X  75  -T-  144  =  163.62. 
163.62  -f-  10  per  cent  =  180  cu.  ft. 

Here  180  cu.  ft.  of  steam  at  80  Ibs.  produce  94  cu.  ft.  of 
air  at  80  Ibs.,  or  i  cu.  ft.  of  air  at  this  pressure  costs 
nearly  2  cu.  ft.  of  steam.  It  should  be  remembered  that 
the  same  ratio  will  not  necessarily  hold  good  for  other 
pressures.  For  lower  air  pressures  the  steam  will  have  a 
little  more  advantage,  and  for  higher  pressures  it  will  have 
a  little  less.  The  mean  effective  resistance  assumed  for 
the  air-cylinder  is  the  theoretical  resistance  with  no  cool- 
ing of  the  air.  In  practice  the  actual  resistance  is  some- 
what less  than  this,  but  the  difference  between  the  air-  and 
the  steam-cards,  or  the  friction  loss  of  the  machine,  is  also 
usually  more  than  10  per  cent,  so  that  few  of  the  common 
compressors  in  use  will  at  their  best  give  any  better  results 
than  the  above. 

The  following  table,  V,  gives  the  horse-power  required  to 
compress  one  cubic  foot  of  free  air  per  minute  to  a  given 
pressure,  also  the  horse-power  required  to  furnish  a  cubic 
foot  of  air  at  the  given  pressure  ;  or,  in  other  words,  the 
power  cost  of  the  operation  of  air-compression  is  exhibited 


92 


COMPRESSED    AIR. 


TABLE  V. 

TABLE  SHOWING  THE  HORSE-POWER  REQUIRED  TO  COMPRESS  AND 
DELIVER  I  CUBIC  FOOT  OF  FREE  AIR  PER  MINUTE  TO  VARIOUS 
GAUGE  PRESSURES,  ALSO  THE  POWER  REQUIRED  TO  COMPRESS 
AND  DELIVER  I  CUBIC  FOOT  OF  AIR  AT  THE  GIVEN  PRESSURE. 


Compressing'  i  Cu.  Ft.  of 

Delivering  i  Cu.  Ft.  per  Min. 

Free  Air  per  Min.  to 
given  Pressure. 

of  Air  Compressed  to  the 
Pressure  given. 

I 

Gauge 

Pressure. 

2 

3 

4 

5 

Compression  at 
Constant 

Compression 
without 

Compression  at 
Constant 

Compression 
without 

Temperature. 

Cooling. 

Temperature. 

Cooling. 

5 

.01876 

.01963 

.02514 

.0263 

IO 

.03325 

'.03609 

.05586 

.06399 

15 

.04507 

.O5O22 

.09105 

.10145 

20 

.05506 

.06283 

.12994 

.14829 

25 

.06366 

.07422 

.17191 

.20043 

30 

.0713 

.08464 

.21678 

•25734 

35 

.0782 

.09425 

.26445 

.31872 

40 

.084305 

.10324 

•31375     - 

.38422 

45 

.08954 

.11166 

.36368 

•45353 

50 

.09508 

.11952 

.41848 

.  52605 

55 

.09936 

.I27O2 

.47112 

.60227 

60 

.  10402 

.13418 

.52855 

.68181 

65 

.10808 

.  14028 

.58612 

.76079 

70 

.11245 

.14718 

.64812 

.8483 

75 

.11629 

•15373 

•70952 

•93795 

80 

.11926 

•I597I 

.76843 

.  02906 

85 

.1224 

•16555 

.83039 

.1231 

90 
95 

•12558 
.12886 

.17096 
.17629 

.89444  . 
.96164 

.2176 
.3148 

IOO 

.13121 

•18153 

1.0243 

.4171 

both  from  the  beginning  and  from  the  termination  of  it. 
From  either  standpoint  the  power  required  is  given  both 
for  isothermal  and  for  adiabatic  compression,  in  the  one 
case  assuming  that  the  air  remains  at  its  initial  tempera- 
ture during  the  compression,  and  in  the  other  case  that  the 
air  as  heated  by  the  compression  is  not  cooled  during  the 
operation.  The  power  required  as  given  in  the  table  is  the 


THE  POWER    COST  OF  COMPRESSED    AIR.       93 

theoretical  power,  and  no  allowance  is  made  for  the  inevi- 
table losses  of  power  that  occur  in  its  actual  application, 
and  of  course  it  makes  no  difference  what  may  be  the 
source  of  the  power,  or  the  economy  with  which  it  may  be 
developed  or  applied.  The  power  employed  may  be 
steam,  with  or  without  cut-off  or  condensation,  water-power, 
electricity,  manual  power,  or  anything  else.  When  the  vol- 
ume of  free  air  required  to  be  compressed  per  minute  is 
known,  or  the  volume  of  air  at  the  given  pressure  required 
to  be  furnished,  the  theoretical  power  required  may  be 
found  by  multiplying  the  number  of  cubic  feet  required  by 
the  power  required  for  i  foot,  as  here  given.  In  the  last 
column  of  the  table  although  the  compression  is  assumed 
to  be  adiabatic  the  air  is  supposed  after  delivery  to  have 
.cooled  to  normal  temperature,  and  to  have  assumed  its 
practically  available  volume,  and  the  i  cu.  ft.  of  com- 
pressed air  represented  in  column  5  is  precisely  the  same  as 
the  i  cu.  ft.  in  column  4. 

In  the  use  of  this  table  the  second  column,  showing  the 
power  cost  of  isothermally  compressing  i  cu.  ft.  of  free  air 
to  the  given  pressure,  represents  the  ideal  and  unattainable, 
but  still  the  only  rational  and  natural,  standard  of  efficiency 
in  air-compression.  Whatever  the  actual  power  employed 
may  exceed  the  values  in  this  column  is  the  irrecoverable 
cost  of  compression.  In  comparing  the  performance  of  a 
steam-actuated  air-compressor  with  this  standard  we  shall 
find  at  least  four  different  sources  of  loss  in  the  operation 
of  compression,  and  all  requiring  some  deduction  from  the 
ideal  efficiency.  Few  persons  in  dealing  with  compressed 
air  recognize  and  make  the  necessary  allowances  and  de- 
ductions for  all  of  these  sources  of  loss,  and  in  consequence 
the  efficiencies  of  the  air-compressors  of  the  day  are  gener- 


94  COMPRESSED   AIR, 

ally  represented  to  be  much  higher  than  they  actually  are. 
In  deploring  the  low  ultimate  efficiencies  in  compressed-air 
systems  we  may  still  find  great  losses  in  the  compression 
end  of  them,  notwithstanding  all  the  boasted  "  modern  im- 
provements." 

The  first  deduction  to  be  made  is  for  the  friction  of  the 
machine,  and  is  accurately  represented  by  the  difference  in 
the  mean  effective  pressures  in  the  air,  and  in  the  steam- 
cylinders,  assuming  the  areas  and  strokes  of  the  two  cylin- 
ders to  be  the  same.  This  difference  is  often  found  to  be 
surprisingly  low.  In  some  large  Corliss  compressors,  where 
the  air-cylinders  are  placed  tandem  to  the  steam-cylinders, 
the  piston-rod  from  the  steam-cylinder  being  continued 
into  the  air-cylinder  to  operate  its  piston,  the  total  loss  of 
power  in  the  friction  of  the  engine  often  ranges  as  low  as 
5  per  cent,  where  the  friction  of  the  same  steam-engine  if 
transmitting  all  of  its  power  through  its  crank-shaft  would 
exceed  10  per  cent.  Compressed  air  evidently  here  has  a 
great  advantage  over  electricity,  and  the  first  power  loss  in 
an  electric  system,  in  driving  the  generator  by  means  of  a 
steam-engine,  and  including  the  friction  of  the  generator,  is 
necessarily  from  two  to  three  times  as  great  as  the  loss  in 
operating  the  air-cylinder  of  a  steam-actuated  compressor 
of  the  best  type.  The  friction  loss  in  the  common  straight- 
line,  direct-acting  air-compressors  may  generally  be  as- 
sumed at  10  per  cent,  and  is  seldom  found  lower  than  that. 
Some  statements  of  air-compressor  efficiencies  are  made 
upon  the  friction  loss  alone,  and  in  the  last-mentioned  in- 
stance the  efficiency  of  the  compressor  would  be  stated  as 
90  per  cent,  with  no  hint  of  any  other  losses,  which  is 
absurd. 

The  second  source  of  loss  to  be  reckoned  with  is  in  the 


THE  POWER    COST  OF  COMPRESSED   AIR.        95 

increase  of  temperature  and  reduction  of  weight  of  air  ad- 
mitted to  the  cylinder  for  compression.  This  loss  is  sel- 
dom recognized,  and  still  more  rarely  made  the  subject  of 
actual  computation.  It  is  difficult  to  determine  it  accu- 
rately, because  it  is  the  one  detail  in  the  cycle  of  operations 
in  air-compression  about  which  the  indicator-diagram  has 
nothing  to  say.  It  is  evident,  however,  that  there  must  be 
some  loss  from  this  source  in  almost  every  case.  As  the 
air  is  always  heated  by  compression,  and  at  best  only  par- 
tially cooled,  the  cylinder  is  heated  by  it,  and  after  continu- 
ous compression  becomes  quite  hot.  Water-jacketing  only 
partially  cools  the  inner  surfaces  of  the  cylinder,  and  some 
parts  of  it  and  the  heads  and  usually  all  of  the  piston  are 
not  cooled  at  all  by  the  "water.  The  air,  which  when 
heated  we  find  to  give  up  its  heat  so  quickly  in  transmis* 
sion,  is  also  heated  with  equal  celerity  when  the  conditions 
are  reversed,  and  it  cannot  pass  through  heated  passages 
into  a  heated  chamber,  which  the  cylinder  is,  without  being 
heated  and  increased  in  volume,  so  that  a  less  weight  or 
actual  quantity  of  air  is  sufficient  to  fill  the  cylinder.  The 
loss  in  many  cases  from  this  source  is  perhaps  light,  but  in 
some  cases  there  can  be  little  doubt  that  it  exceeds  the 
friction  loss  of  the  compressor.  If  air  whose  normal  tem- 
perature is  60°  is  actually  at  120°  at  the  moment  when 
compression  begins  in  the  cylinder,  the  weight  of  air  pres- 
ent is  less  than  90  per  cent  of  the  same  volume  at  its  orig- 
inal  temperature. 

The  third  loss  of  power  in  air-compression  is  due  to  the 
heating  of  the  air  during  the  compression,  and  to  the 
greater  force  required  for  the  compression  on  account  of 
this  heating.  This  is  the  one  source  of  loss  that  is  gener-» 
ally  recognized,  and  too  often  treated  of  as  the  only  one. 


96  COMPRESSED   AIR. 

The  loss  in  this  case  is  represented  by  the  percentage  of 
excess  of  mean  effective  pressure  above  that  required  for 
isothermal  compression.  In  compressing  to  70  Ibs.  the 
M.E.P.  for  isothermal  compression  is  26,  and  for  adiabatic 
compression  it  is  33.73,  and  the  mean  of  the  two  is  29  87. 
The  excess  of  the  adiabatic  above  the  isothermal  is  29.7  per 
cent,  and  the  excess  of  the  mean  above  the  isothermal  is 
still  14.85,  or  say  15  per  cent.  No  compressor  within  my 
knowledge  does  its  compression  to  70  Ibs.  with  less  than  15 
per  cent  of  loss  except  by  devices  that  increase  the  friction 
of  the  machine  or  add  to  the  power  required  or  to  the  cost 
of  operation  in  some  way. 

The  fourth  source  of  power  loss  in  air-compression  lies 
in  the  fact  that  while  the  indicator-cards  show,  as  they  do, 
that  the  M.E.P.  tor  the  compression-stroke  is  above  the 
mean  of  the  isothermal  and  the  adiabatic  pressures,  or 
when  compressing  to  70  Ibs.  more  than  15  per  cent  above 
isothermal  compression,  the  volume  of  free  air  compressed 
is  never  a  cylinderful.  The  figures  in  the  formulas  and  in 
the  tables  are  based  upon  the  assumption  that  a  certain 
volume  of  air  is  compressed,  and  when  applied  to  the  cyl- 
inder of  a  compressor,  the  actual  capacity  of  the  cylinder, 
cr  the  net  area  multiplied  by  the  stroke,  is  the  volume  rep- 
resented. It  is  of  course  the  fact  that  the  volume  actually 
compressed  is  always  somewhat  less  than  this.  There  is  a 
loss  at  each  end  of  the  stroke.  Compression  of  the  air  at 
full  atmospheric  pressure  does  not  begin  precisely  at  the 
beginning  of  the  stroke,  and  all  of  the  air  is  not  expelled 
by  the  piston  at  the  end  of  the  stroke.  It  is  custom- 
ary with  compressor-people  to  say  that  clearance  in  the 
•air-cylinder  at  the  end  of  the  stroke  does  not  mean  loss  of 
power,  but  only  loss  of  capacity,  because  the  power  which 


THE   POWER   COST  OF  COMPRESSED    A  IK.       97 

has  been  expended  in  the  compression  of  the  air  filling  the 
clearance-space  is  returned  to  the  piston  by  the  re-expan- 
sion of  the  air  when  the  piston  makes  its  return  stroke. 
The  clearance  does,  however,  practically  represent  an  actual 
loss  of  power,  or  an  expenditure  of  power  without  any  result, 
because  the  evidence  which  the  clearance  gives  is  so  gener- 
ally ignored,  and  every  stroke  of  the  piston  is  assumed  to 
compress  and  deliver  free  air  to  the  full  capacity  of  the 
cylinder,  which  it  certainly  never  does. 

In  practice  these  four  items  of  loss  of  power  in  compres- 
sion occur  in  different  combinations,  such  as  10,  10,  17, 
10  =  60.5  per  cent  net  efficiency  or  7,  2,  15,  5  =  73.6  per 
cent  net  efficiency.  It  is  safe  to  say  that  the  ultimate  effi- 
ciency never  goes  as  high  as  80  per  cent,  while  it  often  goes 
below  60  per  cent.  If  any  air-compressor  builder  feels 
aggrieved  over  this  statement,  a  fine  opportunity  is  opened 
for  a  demonstration  of  a  higher  efficiency.  Indicator-cards 
from  air-  and  steam-cylinders  are  full  and  conclusive  evi- 
dence as  to  three  of  the  four  items  of  loss  enumerated 
above,  and  it  might  be  profitable  to  make  an  exhibit  of 
these,  and  if  it  proved  to  be  creditable,  we  could  be  gener- 
ous in  our  estimates  of  the  one  concerning  which  no  proof 
seems  to  be  easily  procurable. 


CHAPTER  XI. 

THE  POWER  VALUE  OF  COMPRESSED  AIR. 

THOSE  of  us  who  are  not  wise  enough  to  consider  well 
before  buying  it  what  a  thing  will  be  worth  to  us  are  very 
apt  to  be  looking  it  over  anxiously  after  the  purchase  to 
see  what  sort  of  a  bargain  we  have  got.  As  in  the  last 
chapter  we  learned  the  approximate  power  cost  of  a  cubic 
foot  of  compressed  air  at  a  given  pressure,  we  now  naturally 
want  to  know  what  it  is  worth  to  us.  We  realize  that  in 
the  compression  it  is  costly,  if,  indeed,  we  do  not  think  that 
it  costs  too  much,  and  yet  we  go  on  using  it  more  and  more, 
and  find  profit  in  doing  so.  Our  bargain  is  really  worse 
than  appears  thus  far  ;  for  if  we  take  our  compressed  air 
and  go  to  use  it  as  we  use  steam,  or  if  we  substitute  it  in  a 
place  where  we  have  been  using  steam,  as  in  a  steam-engine, 
we  soon  find  that  a  cubic  foot  of  air  at  any  given  pressure 
is  not  worth  as  much,  in  power,  as  a  cubic  foot  of  steam  at 
the  same  pressure. 

The  accompanying  diagram,  Fig.  18,  shows  how  this  can 
be  so.  Here  we  have  i  volume  of  steam  and  the  same 
of  air,  both  at  100  Ibs.  gauge  pressure,  and  each  success- 
ively expanded  through  several  additional  volumes  until 
the  pressure  of  each  falls  below  that  of  i  atmosphere. 
It  is  readily  seen  that  the  two  expansion-lines  are  very  dif- 
ferent, and  that  the  mean  effective  pressure  of  the  steam  is 
decidedly  higher  than  that  of  the  air.  Thus  i  volume  of 

98 


THE  POWER    VALUE   OF  COMPRESSED   AIR.      99 


steam  at  100  Ibs.  gauge,  represented  by  the  length  of  the 
line  A  i,  reaches  atmospheric  pressure  after  expansion  to 
about  six  and  a  half  times  the  original  volume,  while  the 
same  volume  of  air  drops  to  the  same  pressure  after  expan- 


sion  to  a  little  over  four  times  its  original  volume.  The 
mean  effective  pressure  for  the  steam,  taking  the  whole  ex- 
tent of  the  diagram,  or  cutting  off  at  i  stroke,  is  27.38  Ibs., 
while  the  M.E.P.  for  air  under  the  same  conditions  is  19.51 


IOO  COMPRESSED   AIR. 

Ibs.,  or  only  71  per  cent  of  the  former.  As  with  this  cut- 
off the  terminal  pressures  are  below  the  atmosphere,  the 
entire  mean  effective  pressures  are  not  properly  "  effective" 
or  available  or  comparable.  At  i  cut-off  the  M.E.P.  for 
steam  is  51.93,  and  for  air  it  is  44.19,  or  85  per  cent,  which 
looks  a  little  better  for  the  air,  but  in  this  case  the  terminal 
pressure  of  the  steam  is  n  Ibs.  gauge,  and  some  of  its 
power  is  lost  through  the  exhaust. 

This  diagram  is  equally  applicable  for  any  other  initial 
pressure  below  100,  by  taking  as  the  measure  of  volume 
the  length  of  a  horizontal  line  drawn  from  the  line  AB  to 
the  expansion-line  at  the  given  pressure,  and  taking  each 
repetition  of  this  length  horizontally  as  representing  an 
additional  volume.  Thus  at  60  Ibs.  pressure  i  volume  of 
steam  is  represented  by  i£,  and  2  volumes  would  be  rep- 
resented by  3,  and  at  the  intersection  of  the  vertical  line 
marked  3  we  find  that  the  steam  pressure  has  fallen  to  21 
Ibs.,  which  is  nearly  correct.  One  volume  of  air  at  60  Ibs. 
is  represented  by  about  if  of  the  diagram-spacing,  and  2 
volumes  would  consequently  be  2!  of  the  spaces,  and  here 
we  find  the  air  pressure  to  be  13  -}~>  which  is  the  correct 
terminal  pressure  for  air  at  60  Ibs.  cut-off  at  £  stroke,  or 
expanded  to  double  the  volume.  We  may  take  any  sec- 
tion of  this  diagram  as  representing,  theoretically,  an  indi- 
cator-card either  for  steam  or  air,  but  we  cannot  take  both 
the  steam-  and  the  air-cards  and  compare  them  by  placing 
one  upon  the  other,  because  the  lengths  of  the  two  cards 
will  not  coincide. 

Fig.  19  is  a  theoretical  card,  scale  40,  showing  both 
steam  and  air  expanded  to  atmospheric  pressure  at  the  end 
of  the  stroke.  In  this  case  the  air-line  is  outside  of  and 
above  the  steam-line,  and,  of  course,  represents  a  higher 


THE  POWER    VALUE   OF  COMPRESSED   AIR.    IOI 


mean  effective  power,  but  it  is  at  the  expense  of  a  much 
larger  initial  volume.  The  M.E.P.  for  air  filling  a  cylinder  at 
an  initial  pressure  of  zoo  Ibs.  for  a  sufficient  portion  of  the 


stroke  and  then  expanding  (without  loss  or  gain  of  heat)  so 
that  it  reaches  atmospheric  pressure  at  the  end  of  the 
stroke  will  be  41.6  Ibs.  The  M.E.P.  for  steam  under  the  same 
conditions  will  be  32.46.  The  volume  of  air  used  will  be 


102  COMPRESSED   AIR. 

•2353>  while  the  volume  of  steam,  will  be  .1471  If  the  air 
gave  the  same  M.E.P.  in  proportion  to  its  volume,  it  would 
be .  147 1  :  2353  :  :  32.46  :  51.9,  instead  of  41.6,  and  the  greater 
comparative  efficiency  of  steam  under  the  conditions  is 
41.6  :  51.9  :  :  i  :  1.247,  or  nearly  25  per  cent. 

As  the  expansion  of  the  air  here  exhibited  is  adiabatic, 
its  temperature,  at  least  for  the  latter  portion  of  the  expan- 
sion, would  be  below  that  of  the  cylinder  containing  it,  and 
the  air  would  be  heated  and  expanded,  rather  than  cooled, 
by  its  surroundings  ;  so  that  there  need  be  no  apprehension 
that  the  expansion-line  would  be  below  the  theoretical,  or 
that  there  might  be  still  some  lurking  losses  to  arise  and 
confront  us.  The  essential  difference  in  an  engine  or 
motor  to  be  driven  by  compressed  air  instead  of  steam  is  a 
later  cut-off  for  the  same  initial  pressure.  This  later  cut- 
off develops  the  paradox  that  although  air  has  less  available 
power  than  steam,  volume  for  volume,  the  same  cylinder  with 
the  same  pressure  will  develop  more  power  with  air  than 
with  steam,  both  being  used  at  the  point  of  highest  efficiency. 

I  offer  herewith  a  table,  VI,  showing  the  mean  effective 
and  terminal  pressures  for  both  steam  and  air  at  various 
points  of  cut-off  and  for  different  gauge  pressures  from  50  to 
100.  Gauge  pressures  are  given  throughout  except  when 
below  atmosphere  when  the  absolute  pressures  are  given  in 
italics.  It  is  thought  that  in  this  way  the  table  will  be  more 
serviceable  to  the  general  mechanic  than  if  the  absolute 
pressures  were  given  throughout.  Nothing  is  said  of  the 
initial  temperature  of  the  air,  as  that  would  not  affect  the 
rate  of  expansion  or  the  mean  effective  pressure. 


THE  POWER    VALUE   OF  COMPRESSED   AIR.    1 03 


TABLE  V! 

TABLE  OF  MEAN  EFFECTIVE  AND  TERMINAL  PRESSURES  OF  STEAM  AND 
AIR  AT  VARIOUS  POINTS  OF  CUT-OFF  AND  FOR  DIFFERENT  GAUGE- 
PRESSURES  FROM  5O  TO  TOO  LBS. 

All  pressures  given  in  the  table  are  gauge  pressures,  except  where  they  fa.:. 
below  atmosphere,  when  the  absolute  pressures  are  given  and  printed  in  full  face. 

INITIAL   PRESSURE   50   LBS. 


Point 
of 
Cut  off. 

Mean  Steam 
Pressure. 

Mean  Air 
Pressure. 

Terminal 
Steam 
Pressure. 

Terminal 
Air 
Pressure. 

•05 

12.  12 

8.87 

2.69 

-95 

A 

14-39 

10.8 

3-41 

i-3i 

.10 

5-44 

1.2 

5.63 

2-54 

i 

8.95 

4.51 

7-13 

3-47 

•  15 

10.18 

7.62 

8.65 

4-49 

A 

16-55 

11.96 

10-97 

6.14 

.20 

17.9 

13.84 

"•75 

6.74 

.25 

22.83 

18.45 

14.9 

9  23 

•30 

27.11 

23.05 

3-08 

"93 

i 

29.66 

25.84 

5.22 

1383 

•35 

30.86 

27.17 

6-3 

14.82 

1 

32.56 

29.07 

7-92 

1-34 

.40 

34-15 

30.87 

9-55 

2.88 

•45 

37-03 

34-18 

12.84 

4.11 

•  50 

39-54 

37-12 

16.12 

7-49 

.60 

43.61 

41.98 

22.77 

16.66 

1 

44.44 

42.99 

24.44 

18-53 

* 

45.67 

44.52 

27.24 

21-73 

.70 

46.54 

45-6 

29-49 

24-33 

•75 

47.64 

46.98 

32.88 

28.34 

.80 

48.52 

48.08 

36.27 

32-47 

* 

49-43 

49.26 

41.4 

38.85 

.90 

49.64 

49-53 

43-H 

41-03 

104 


COMPRESSED    AIR. 
TABLE  VI.— (Continued.) 


INITIAL    PRESSURE   60   LI 


Point 
of 
Cut-off. 

Mean  Steam 
Pressure. 

Mean  Air 
Pressure. 

Terminal 
Steam 
Pressure. 

Terminal 
Air 
Pressure. 

•  05 

13-99 

10.23 

3-i 

I.I 

A 

l.6i 

12.46 

3-93 

I-5I 

.10 

8.58 

3.69 

(.49 

2-93 

i 

12.64 

7-51 

8.22 

4.01 

.15 

16.37 

II.  I 

9  99 

5-21 

ft 

21.41 

16.11 

12.66 

7-08 

.20 

22.96 

17-7 

13-56 

7-77 

.25 

28.75 

23-6 

2.19 

10.65 

•30 

33-59 

28.9 

5-87 

13-77 

i 

36.54 

32.13 

8-34 

.96 

•  35 

37-92 

33-66 

9-58 

2-33 

I 

39.87 

35-85 

II.  8 

3-85 

.40 

41.71 

37-93 

13-22 

5-64 

•  45 

45-03 

41-75 

17.1 

10.71 

•  50 

47-94 

45.14 

20.91 

13-26 

.60 

52.62 

50.75 

28.59 

21-53 

i 

53.58 

5I-92 

30.51 

23.69 

I 

55-01 

53-67 

33-74 

27.94 

.70 

56.01 

54-93 

36.34 

30-39 

•  75 

57.28 

56.52 

40.24 

35.01 

,80 

58.29 

57-79 

44.06 

39.78 

1 

59-34 

59-15 

50.07 

47.14 

.90 

59.58 

59-46 

52.05 

49.65 

THE  POWER    VALUE   OF  COMPRESSED   AIR.   10$ 
TABLE  VI.—  (Continued.) 

INITIAL   PRESSURE  70  LBS. 


Point 
of 
Cut-off. 

Mean  Steam 
Pressure. 

Mean  Air 
Pressure. 

Terminal 
Steam 
Pressure. 

Terminal 
Air 
Pressure. 

.05 

1.  06 

ii.  6 

3-52 

1.23 

TV 

3-82 

14.12 

4.46 

I.7I 

.IO 

11-73 

6.19 

7.36 

3.32 

i 

16.33 

10.51 

9-32 

4-54 

•15 

20.55 

14.58 

11.32 

5-88 

& 

26.26 

20.25 

M  37 

8.03 

.20 

28.02 

22.06 

•  37 

8.81 

•25 

34-47 

28.74 

4-49 

12.07 

•30 

40.07 

34-75 

6.65 

.6 

i 

43-41 

38.41 

11-45 

3-og 

•  35 

44-97 

40.15 

12.86 

4-38 

I 

47.19 

42.63 

14.98 

6.36 

.40 

49.27 

44-99 

17.1 

8-39 

•  45 

53-04 

49-31 

21.38 

12.  6l 

•50 

56.33 

53.16 

25.69 

17 

.60 

61.64 

59-51 

34-4 

26.4 

I 

62.73 

60.84 

36.58 

28.85 

1 

64-34 

62.83 

40.24 

33-03 

.70 

65.48 

64.25 

43-19 

36.44 

•  75 

66.92 

66.05 

47.61 

41.68 

.80 

68.07 

67.5 

52-05 

47-08 

1 

69.26 

69.03 

58.75 

55-43 

.90 

69-53 

69.38 

60.99 

58.27 

io6 


COMPRESSED   AIR. 
TABLE  VI.— (Continued.} 


INITIAL   PRESSURE    80   LBS. 


Point 
of 
Cut-off. 

Mean  Steam 
Pressure. 

Mean  Air 
Pressure. 

Terminal 
Steam 
Pressure. 

Terminal 
Air 
Pressure. 

•05 

2.72 

12.96 

3-93 

1-39 

ft 

6.04 

.78 

4.98 

1.92 

,IO 

14.87 

8.68 

8.22 

3.7I 

i 

20.  01 

13-51 

10.42 

5.08 

•15 

24.73 

18.06 

12.65 

6-57 

A 

31.12 

24.4 

1.04 

8.97 

.20 

33.08 

26.6 

2.18 

9-85 

•25 

40.29 

33.89 

6.78 

13-49 

•30 

46.55 

40.61 

"•43 

2.44 

i 

50.28 

44.69 

14.56 

5.22 

•  35 

52.03 

46.64 

16.14 

6.66 

1 

54.51 

49.41 

18.5 

7.88 

.40 

56.83 

52.05 

20.88 

11.14 

•45 

61.04 

56.9 

25.66 

15-86 

•50 

64.72 

61.18 

30.48 

20.  8l 

.60 

70.76 

68.28 

40.21 

31-27 

f 

71.87 

69.76 

42.65 

34.01 

I 

73-68 

71.99 

46.74 

38.68 

.70 

74-95 

73-57 

50.03 

42.49 

•  75 

76.56 

75-59 

54-97 

48.35 

.80 

77-84 

77-2 

59-94 

54.38 

1 

79-17 

78.92 

67.43 

63.81 

.90 

79-47 

79-31 

69-93 

66.89 

THE  POWER    VALUE   OF  COMPRESSED   AIR.   IO? 
TABLE  VI.— (Continued.) 

INITIAL  PRESSURE  90   LBS. 


Point 
of 
Cut-off. 

Mean  Steam 
Pressure. 

Mean  Air 
Pressure. 

Terminal 
Steam 
Pressure. 

Terminal 
Air 
Pressure. 

•05 

4-59 

M-33 

4-34 

i  -54 

A 

8.25 

2.95 

S-Si 

2.12 

.10 

18.02 

11.17 

9.09 

4.1 

i 

23.7 

16.52 

11.51 

S-6I 

.15 

28.92 

21.55 

13.98 

7.26 

A 

35.97 

28.55 

2-73 

9.92 

'     .20 

38.15 

30.78 

3-99 

10.88 

•25 

46.11 

39-°4 

9-07 

14.91 

•30 

53-02 

46.46 

14.22 

4.27 

* 

57-17 

50.98 

17.67 

7-35 

•35 

59.08 

53-13 

19.42 

8-95 

t 

61.82 

56.2 

22.03 

"•39 

.40 

64.4 

59-" 

24-65 

13.88 

•45 

69.05 

64.45 

29.95 

19.11 

•  50 

73-11 

69.19 

33-27 

24.56 

.60 

79.67 

77-05 

46.02 

36.14 

1 

81.02 

78.69 

48.72 

39.16 

1 

83.01 

81.14 

53.23 

44-33 

.70 

84.42 

82.9 

56.88 

48.54 

•75 

86.19 

85.12 

62.34 

55-02 

.80 

87.61 

86.91 

67.83 

61.69 

1 

89.08 

88.81 

76.1 

72 

.90 

89.42 

89.24 

78.88 

75-52 

io8 


COMPRESSED   AIR. 
TABLE  VI.— {Continued.) 


INITIAL    PRESSURE    IOO    LBS. 


Point 
of 
Cut-off. 

Mean  Steam 
Pressure. 

Mean  Air 
Pressure. 

Terminal 
Steam 
Pressure. 

Terminal 
Air 
Pressure. 

•05 

6-45. 

.69 

4-76 

!.69 

* 

10.24 

4.  II 

6.03 

2.32 

.IO 

21.  l6 

13.66 

9-95 

4-49 

i 

27.38 

I9-5I 

12.  6l 

6.15 

.15 

33-1 

25.03 

•31 

7-95 

A 

40.83 

32.71 

4.42 

10.89 

.20 

43.21 

35-14 

5-79 

11.92 

•25 

51-93 

44.19 

11.36 

1-33 

•30 

59-5 

53-32 

17 

6.  ii 

i 

64.02 

57.26 

20.78 

9.48 

•  35 

66.14 

59.62 

22.69 

11.23 

I 

69.14 

62.98 

25.56 

13.89 

.40 

71.96 

66.16 

28.43 

16.64 

•  45 

77.05 

72.O2 

34-23 

22.36 

•50 

81.5 

77.21 

40.06 

28.33 

.60 

88.69 

85-82 

51-83 

41.01 

f 

90.15 

87.61 

54-79 

44-32 

1 

92.19 

90.32 

59-73 

49-97 

.70 

93.89 

92.22 

63.72 

54-59 

•75 

95.83 

94.66 

69.7 

61.69 

.80 

97.38 

96.61 

75-72 

68.99 

1 

98.99 

98.7 

84.78 

80.28 

.90 

99.36 

99.17 

87.82 

84.14 

CHAPTER  XII. 
COMPRESSED-AIR  TRANSMISSION. 

THE  accompanying  table,  VII,  gives  the  actual  volume  of 
air  passing  through  a  pipe  of  given  diameter  when  the  linear 
velocity  of  flow  is  known.  This  is  merely  a  convertible 
table  of  pipe  capacity,  and  will  be  useful  as  such  in  deter- 
mining the  size  of  pipe  best  adapted  for  a  given  service, 
and  it  has  nothing  to  do  with  the  conditions  which  may 
determine  the  rate  of  transmission. 

While  in  considering  the  operation  of  air-compression  we 
base  our  computations  upon  the  volume  of  free  air  com- 
pressed, it  is  better  in  questions  relating  to  the  transmission 
of  the  air  to  consider  the  actual  volume  of  the  air  during 
the  transmission,  or  usually  either  at  the  beginning  or  at 
the  completion  of  the  transmission.  As  air  in  transmission 
soon  attains  the  temperature  of  the  pipe  and  its  surround- 
ings, its  temperature  need  not  generally  be  taken  into  ac- 
count as  affecting  the  volume.  The  volume  of  free  air 
transmitted  may  be  assumed  to  be  directly  as  the  absolute 
pressure  or  the  number  of  atmospheres  to  which  the  air  is 
compressed.  Thus  if  the  air  transmitted  be  at  75  Ibs.,  or 
6  atmospheres,  the  actual  volume  of  free  air  transmitted 
will  be  six  times  the  volume  given  in  the  body  of  the  table. 
For  comparing  cases  of  transmission  the  linear  velocity  of 
flow  is  generally  adopted,  and  is  the  more  convenient  form 
of  statement.  It  is  generally  considered  that  for  econom- 
ic^ 


no 


COMPRESSED   AIR, 


COMPRESSED-AIR    TRANSMISSION.  Ill 

ical  transmission  the  actual  velocity  in  main  pipes  should 
not  exceed  20  feet  per  second.  It  would  be  well  if  more 
attention  were  given  to  the  capacities  of  the  distributing- 
pipes  employed.  In  practice  it  often  occurs  that  while  the 
main  pipe  is  large  enough  for  the  transmission,  the  smaller 
pipes,  or  hose,  through  which  the  air  is  finally  transmitted 
to  the  individual  machines  are  too  small,  and  velocities  n 
high  as  50  feet  per  second  are  not  infrequently  met  with. 

Compressed  air  has  usually  to  be  transmitted  a  greater  or 
less  distance  from  the  compressor  to  the  place  where  it  is 
used,  and  in  computing  the  cost  or  the  economy  of  opera- 
tions in  which  compressed  air  is  employed  it  becomes 
necessary  to  consider  the  friction  of  the  air  in  the  pipes, 
and  the  power  lost  in  overcoming  it.  Upon  this  point 
some  very  extravagant  and  widely  erroneous  ideas  have 
been  quite  generally  disseminated.  The  popular  impres- 
sion, is  that  great  power  losses  are  inseparable  from  the 
transmission  of  air  through  pipes.  The  facts  are  quite  dif- 
ferent from  this.  It  seems  to  be  certain  that  power  may  be 
transmitted  by  compressed  air  for  considerable  distances 
with  less  loss  than  by  any  other  known  means.  We  can- 
not  do  anything  for  nothing,  and  of  course  there  is  some 
loss  of  power  in  the  transmission  of  compressed  air,  but  in 
general  practice  thus  far,  unless  the  piping  system  has  been 
outrageously  bad  and  inadequate,  the  losses  by  transmission 
have  not  been  worth  considering.  The  distances  traversed 
have  usually  not  been  great  enough  to  make  the  loss  a 
serious  one,  or  at  all  to  be  compared  with  the  losses  of 
power  through  the  heating  of  the  air  in  compression,  or 
through  its  loss  of  volume  by  the  cooling  of  re-expansion 
when  the  air  was  finally  employed.  But  as  compressed- 
air  practice  develops  we  want  longer  lines,  and  then  the 


112  COMPRESSED   AIR. 

question  of  transmission  rises  to  greater  importance.  We 
want  to  convey  air  long  distances  for  the  purpose  of  em- 
ploying unused  and  now  worthless  water-powers.  We 
want  long  lines  of  piping  for  supplying  street-cars  with  the 
means  of  propulsion  and  for  the  general  distribution  of 
power  and  the  general  application  of  compressed  air  to  its 
multifarious  uses  in  large  cities.  We  want  to  convey  nat- 
ural gas  from  the  wells  where  it  flows  to  the  towns  and 
cities,  where  it  may  be  used  to  the  best  advantage. 

When  we  come  to  look,  up  the  formulas  to  depend  upon 
for  our  computations,  we  do  not  find  any  that  are  satis- 
factory and  reliable.  The  best  that  may  be  said  of  the  best 
of  them,  and  that  with  caution,  is  that  they  are  approxi- 
mately correct,  but  we  know  that  they  must  be  in  error  in 
some  particulars.  The  available  data  in  this  line  are 
meagre  and  chaotic,  inconsistent  and  self-evidently  unre- 
liable. It  is  perhaps  too  much  to  expect  that  close  accu- 
racy can  ever  be  attained  in  these  computations.  There  are 
too  many  factors  in  the  case,  and  a  little  variation  in  any 
one  may  make  a  great  difference  in  the  result.  A  state- 
ment of  the  friction  in  the  case  of  compressed  air  flowing 
through  a  pipe  involves  at  least  all  of  the  following  factors  : 
Unit  of  time,  volume  of  air,  pressure  of  air,  diameter  of 
pipe,  length  of  pipe,  and  the  difference  of  pressure  at  the 
ends  of  the  pipe,  or  the  head  required  to  maintain  the  flow. 
Neither  of  these  factors  can  be  allowed  its  independent  and 
absolute  value,  but  is  subject  to  modifications  in  deference 
to  its  associates.  The  flow  of  air  being  assumed  to  be 
uniform  at  the  entrance  to  the  pipe,  the  rate  of  flow  is  not 
constant  for  the  whole  length  of  the  pipe,  nor  indeed  for 
any  point  but  the  beginning  of  it.  As  the  air  may  be  said 
to  carry  in  itself,  in  its  elasticity,  its  own  means  of  propul- 


COMPRESSED-AIR    TRANSMISSION.  11$ 

sion,  some  of  which  it  is  using  as  it  goes  along,  it  is  con- 
stantly losing  some  of  its  pressure,  and  its  volume  is  there- 
fore constantly  increasing.  If  the  quantity  of  air  entering 
the  pipe  is  to  continue  to  flow  through  it,  the  linear  velocity 
of  flow  must  be  constantly  accelerated  on  account  of  this 
increase  of  volume.  This  also  modifies  the  use  of  the 
length  of  the  pipe  as  a  constant  factor.  It  would  be  very 
natural  to  assume,  as  in  the  formulas  in  general  use  it  is 
assumed,  that  if  a  certain  head  were  sufficient  to  main- 
tain a  certain  flow  for  a  given  length  of  pipe,  double  the 
head  would  be  sufficient  for  double  the  length,  But  that 
could  not  be  so  ;  for  in  the  second  length  all  the  head  that 
propelled  the  air  through  the  first  length  has  disappeared, 
and  the  volume  is  now  greater  through  the  loss  of  that 
pressure,  and  the  velocity  is  now  greater,  and  it  must  require 
more  additional  head  for  the  second  length  than  was  re- 
quired for  the  first.  So  of  the  other  important  factor,  the 
diameter  of  the  pipe.  The  actual  area  of  section,  or  the 
apparent  capacity  of  the  pipes,  is,  of  course,  directly  as  the 
square  of  the  diameter,  but  the  volume  of  air  transmitted 
for  given  length  and  head  will  not  be  in  any  such  ratio. 
The  surface  resistance  of  the  interior  is  proportionately 
much  greater  in  the  smaller  pipe.  While  the  area  is  as  the 
square  of  the  diameter,  the  periphery  is  directly  as  the 
diameter.  The  greatest  distance  of  any  portion  of  the  air 
from  the  periphery  being  less  in  the  smaller  pipe,  the  viscos- 
ity of  the  air  counts  for  more.  For  volumes  in  proportion 
to  the  areas  of  the  pipes  a  i-inch  pipe  will  require  for  a 
given  length  more  than  three  times  as  much  head  as  a  2- 
inch  pipe. 

Then  besides  the  fluctuating  values  of  these  fickle  fac- 
tors there  is  that  other  factor,  unrecognized  in  our  compu- 


114  COMPRESSED   AIR. 

tations,  but  arrogantly  assertive  in  practice — the  condition 
of  the  pipe  itself.  The  actual  diameter  of  wrought-iron 
pipe,  especially  in  the  smaller  sizes,  is  different  from  the 
nominal  diameter.  Some  pipe  is  smooth,  and  some  has 
seams  and  blisters.  The  pipe  may  be  straight,  or  it  may  be 
crooked  and  have  numerous  elbows.  Everybody  knows 
that  elbows  are  unpleasant  things  to  encounter.  Tables 
have  been  published  of  the  effect  of  elbows  in  retarding 
the  flow  of  compressed  air.  One  of  these  tables,  copy- 
righted, is  before  me,  and  from  it  I  gather  that  eight  or  ten 
i -inch  elbows  have  a  retarding  effect  equal  to  one  length 
of  pipe,  so  that  if  the  table  is  to  be  believed,  elbows  are  not 
as  obstructive  as  they  are  commonly  supposed  to  be.  If 
we  say,  without  copyright,  that  a  single  elbow  is  equal  to  a 
length  of  pipe  we  will  be  nearer  right  than  the  table. 

No  table  or  formula  can  make  allowances  for  foreign 
substances  or  obstructions  in  the  pipe,  and  it  seems  unnec- 
essary to  call  attention  to  the  necessity  of  thoroughly  blow- 
ing out  the  pipe  before  it  is  put  to  use.  Long  lines  oi 
pipe  are  sometimes  laid  through  a  variety  of  rough  coun- 
try, and  before  the  pipe  is  coupled  up  many  things  get 
into  it  that  have  no  right  to  be  there.  In  pipe-lines  for 
transmitting  natural  gas  it  has  been  the  practice-  before  the 
pipes  were  put  to  service  to  turn  on  the  full  pressure  of  the 
gas  and  blow  out  the  pipe.  The  pressure  in  such  cases  is 
often  as  high  as  four  or  five  hundred  pounds  to  the  inch, 
and  under  such  a  force  the  pipe  is  usually  quite  effectually 
cleared,  stones,  sticks,  leaves,  squirrels,  rats,  and  snakes 
having  sometimes  been  ejected. 

And  here  it  might  be  proper  to  say  a  word  about  the 
importance  of  the  unimportant.  It  is  the  general  practice 
of  pipers  when  running  a  line  of  pipe  for  air  or  water  or 


COMPRESSED-AIR    TRANSMISSION.  1 15 

steam  to  put  the  white  lead,  or  whatever  may  be  used  as  a 
cement  for  the  joint,  into  the  coupling  or  elbow  or  other 
"  female  "  fitting,  wiping  it  around  and  filling  the  threads 
with  it,  and  then  wheri  the  end  of  the  pipe  or  "the  "  male" 
thread  is  screwed  into  it,  none  of  the  cement  is  left  upon 
the  outside,  and  a  neat  and  clean-looking  job  is  the  result. 
The  trouble  in  the  case  is  that  the  job  would  not  be  so 
neat  and  cleanly  looking  if  it  could  be  seen  from  the  in- 
side. As  the  pipe  end  is  screwed  into  the  fitting  the  ce- 
ment that  does  not  remain  in  the  threads — and  not  much 
of  it  does  remain  in  the  threads — is  carried  forward  before 
the  end  of  the  pipe,  and,  when  the  pipe  is  screwed  home, 
remains  there  and  hardens,  often  rising  above  the  inner  sur- 
face of  the  pipe  enough  to  cause  a  considerable  stricture  or 
reduction  of  pipe  area.  Now,  if  instead  of  this  the  cement- 
ing material  is  put  upon  the  male  thread  when  the  pipe  is 
screwed  in,  all  that  is  not  taken  up  by  the  threads  remains 
on  the  outside  of  the  pipe,  instead  of  inside  ;  and  although 
it  does  not  look  as  neat  as  by  the  other  practice,  and  en- 
tails the  labor  of  wiping  off  the  joint,  we  know  that  the  in- 
side of  the  pipe  is  clear. 

FORMULA    FOR   THE    FRICTION   OF   AIR   IN   PIPES. 

D  =  Diameter  of  pipe  in  inches  ; 

L  —  Length  of  pipe  in  feet ; 

V '  =  Volume  of  air  delivered  in  cubic  feet  per  minute  ; 
H '=  Head  or  difference  of  pressure  required  to  overcome 
friction  and  maintain  the  flow. 


Il6  COMPRESSED    AIR. 

v=\ 


10,000 H' 
L  =          TFs        • 


VALUES   OF   a   FOR   DIFFERENT    NOMINAL   DIAMETERS   OF 
WROUGHT-IRON    PIPE. 


•    -^3 

e 

•/^ 

.787 

10" 

.662 

4  ".. 

84 

12"  ,. 

.    o    .         I     26 

q   "    . 

Q-iA 

16" 

6  5 

6   "  

,  ,     I 

20"     . 

24".. 

It  will  be  noticed  that  the  values  of  a  for  the  i\"  and 
the  ii"  pipes  are  not  consistent  with  the  values  given  for 
the  other  sizes  of  pipe.  This  is  in  recognition  of  the  actual 
diameters  of  those  two  nominal  sizes  of  wrought-iron  pipe, 
which  are  1.38"  and  1.61"  .respectively. 

FIFTH    POWERS   OF   D. 


I  ".... 

I 

3".... 

243 

8".... 

32,768 

li".... 

3-°S 

3i".... 

525 

10".... 

100,000 

I*".... 

7-59 

4".... 

1,024 

12".... 

248,832 

2  ".... 

32 

5  "...- 

3,125 

20".... 

3,200,000 

**".-.. 

97-65 

6  ".... 

7,776 

24"..-. 

7,962,624 

COMPRESSED-AIR    TRANSMISSION.  117 

Two  or  three  examples  are  offered  showing  the  applica- 
tion of  the  above  formulas,  although  their  use  should  be 
sufficiently  evident  to  anyone  capable  of  making  the  com- 
putations. The  volume,  V,  is  of  course  the  actual  volume  of 
the  air  as  it  flows  through  the  pipe  under  pressure,  and  not 
the  volume  of  free  air. 

Say  that  we  wish  to  transmit  1200  cu.  ft.  of  free  air  per 
minute  at  75  Ibs.  gauge  pressure,  or  6  atmospheres,  through 
a  4"  pipe  for  1000  ft.,  what  additional  pressure  or  head  will 
be  required  to  overcome  the  friction  and  maintain  the  flow 
of  air  ?  1200  cu.  ft.  of  free  air  -f-  6  =  200  cu.  ft.  at  75  Ibs. 
gauge.  Then 

200"  X  1000  .     , 

H =  — —  =  4.65  Ibs.  head  required. 

10,000  X  1024  X  .84 

Having  a  4"  pipe  1000  ft.  long  and  a  head  of  5  Ibs.,  what 
volume  of  air  will  be  transmitted  per  minute  ? 


r= 


The  volume  of  free  air  in  this  case  will  be  dependent 
upon  the  pressure  during  the  transmission.  If  this  207.38 
cu.  ft.  were  under  a  pressure  of  60  Ibs.  gauge,  or  5  atmos- 
pheres, the  volume  of  free  air  would  be  207.38  X  5  =  1036.9 
cu.  ft.  If  the  pressure  were  90  Ibs.  gauge,  or  7  atmospheres, 
the  volume  of  free  air  would  be  207.38  X  7  =  1451.66  cu.  ft. 

Having  2000  cu.  ft.  of  free  air  per  minute  compressed  to 
roo  Ibs.  gauge,  through  what  length  of  6"  pipe  may  it  be 
transmitted,  losing  10  Ibs.  pressure  in  the  transmission  ? 
Here  the  terminal  pressure  would  be  90  Ibs.  gauge,  or  7  at- 


II  8  COMPRESSED    AIR. 

mospheres,  and  the  volume  would  consequently  be  2000  -4-  7 
=  285.7  cu.  ft.     Then 

T       10,000  X  7776  X  i  X  10 
L  =  =  95^6  ft. 


Having  1500  cu.  ft.  of  free  air  per  minute  to  transmit  a 
distance  of  2000  ft.,  the  air  being  at  80  Ibs.  gauge,  and  wish- 
ing to  deliver  it  at  75  Ibs.,  what  should  be  the  diameter  of 
the  pipe?  Here  we  have  a  head  of  5  Ibs.,  and  the  air  is 
delivered  at  a  pressure  of  6  atmospheres,  so  that  the  deliv- 
ery-volume will  be  1500  cu.  ft.  -T-  6  =  250  cu.  ft.  Then  we 
have 

_5         250"  X  2000 
Da  —  —  --  =  2500  in. 
10,000  X  5 

This  is  the  only  case  where  the  fifth  power  can  possibly 
make  any  trouble  for  us,  and  by  referring  to  the  following 
table  of  values  of  D^a  for  the  regular  sizes  of  pipe  the 
necessity  of  struggling  with  the  fifth  root  is  avoided. 

VALUES  OF  D6a. 


T  " 

35 

s". 

2,918.75 

i*". 

-'.525 

6" 

7,776 

T*", 

-  5-°3 

S" 

36,864 

2  ", 

,  18.08 

10". 

120,000 

2i"- 

63.47 

T7" 

3i3,528 

3  " 

177-4 

16". 

1,405,091 

3l" 

4i3-2 

on" 

4,480,000 

4". 

860.2 

24". 

ii,545»8°5 

Our  answer  above  being  2,500,  we  note  that  it  is  less  than 
2,918.75,  the  value  of  D*a  for  5"  pipe,  so  that  a  5"  pipe 
will  be  a  little  larger  than  is  required  by  the  conditions,  and 


COMPRESSED  -A  IR    TRA  tfSMISSION. 


is  the  size  of  pipe  that  should  be  used.  We  may  verify  this 
by  assuming  a  5"  pipe  and  computing  what  head  would  be 
required,  the  other  conditions  remaining  unchanged. 


250   X    2000 


10,000  X  3125  X  .934 


=  4.28. 


As  this  head  is  somewhat  smaller  than  5,  the  given  head, 
this  also  shows  that  a  5"  pipe  would  be  a  trifle  larger  than 
would  be  required  by  the  conditions,  while  a  4"  pipe  would 
be  much  too  small. 

The  pressures  to  which  air  is  compressed  do  not  in 
practice  always,  or  generally,  occur  in  even  atmospheres. 
The  following  table,  VIII,  will  be  found  convenient  in  as- 
certaining the  actual  volume  of  compressed  air  at  any  given 
pressure  if  the  volume  of  free  air  is  given,  or  vice  versa. 

TABLE  VIII. 

TABLE  OF  THE  RELATIVE   VOLUMES  OF  COMPRESSED  AIR*AT  VARIOUS 
PRESSURES. 


Volume  of 

Volume  at 

Volume  of 

Volume  at 

Gauge 
Pressure. 

Free  Air 
for  t  Cu.  Ft. 
at  given 

given  Pressure 
for  i  Cu.  Ft. 
of 

Gauge 
Pressure. 

Free  Air 
for  i  Cu.  Ft. 
at  given 

given  Pressure 
for  i  Cu.  Ft. 
of 

Pressure. 

Free  Air. 

Pressure. 

Free  Air. 

O 

I 

I 

45 

4.061 

.2462 

I 

1.068 

.9356 

50 

4.401 

.2272 

2 

I.I36 

.8802 

55 

4-74 

.2109 

3 

1.204 

.8305 

60 

5-08 

.1967 

4 

1-273 

.7861 

65 

5-421 

.1844 

5 

1-34 

.7462 

70 

5.762 

•1735 

10 

1.68 

•  5951 

75 

6.IO2 

.1638 

15 

2,02 

•4949 

80 

6.442 

•1552 

20 

2.36 

.4236 

85 

6.782 

.1474 

25 

2-7 

.3703 

90 

7-122 

.1404 

30 

3-041 

.3288 

95 

7.462 

.1340 

35 

3-381 

-2957 

100 

7.802 

.1281 

40 

3-72 

.2687 

120  COMPRESSED   AIR. 

The  second  column  in  the  above  table  gives  the  volume 
of  free  air  for  i  cu.  ft.  of  compressed  air  at  a  given  pressure, 
and  this  value  may  be  used  as  a  multiplier  for  any  number 
of  cubic  feet  at  given  pressure  to  ascertain  the  equivalent 
volume  of  free  air. 

Having  550  cu.  ft.  of  air  at  80  Ibs.  pressure,  what  will  be 
the  volume  of  free  air  ? 

550  X  6.442  =  3548.1  cu.  ft. 

The  third  column  in  the  table  gives  the  volume  of  air  at 
any  given  pressure  for  i  cu.  ft.  of  free  air,  and  this  value 
also  may  be  used  as  a  multiplier  for  any  number  of  feet  of 
free  air  to  ascertain  its  volume  after  compression  to  a  given 
pressure. 

If  we  have  1750  cu.  ft.  of  free  air,  what  will  be  its  volume 
when  compressed  to  65  Ibs.  ?. 

1750  X  .1844  =  322.7  cu.  ft. 

The  following  table,  IX,  of  the  head  or  additional  press- 
ure required  to  overcome  friction  in  the  flow  of  air  in  pipes 
has  been  computed  by  the  preceding  formulae.  It  is  be- 
lieved to  be  correct  and  reliable  as  far  as  it  goes,,  and  should 
be  a  convenience  in  many  cases  of  compressed-air  trans- 
mission for  rock  drills  and  similar  uses.  A  table  covering 
all  the  various  pressures  and  conditions  in  general  practice 
would  be  too  voluminous  to  offer  here. 

As  we  have  before  remarked,  so  many  conditions  may 
combine  to  modify  the  specific  case  of  transmission  that 
both  the  formulas  and  the  table  here  given  can  have  only  a 
rough  and  general  application  and  a  provisional  usefulness 
until  something  better  appears. 


COMPRESSED-AIR    TRANSMISSION. 


121 


TABLE  IX. 

TABLE  OF  HEAD  OR  ADDITIONAL  PRESSURE  REQUIRED  TO  DELIVER 
AIR  AT  75  LBS.  GAUGE  PRESSURE  THROUGH  PIPES  OF  VARIOUS 
SIZES  AND  LENGTHS. 

i-inch  Pipe. 


Linear 

Volume 

Length  of  Pipe  in  Feet. 

of 

Feet  per 

Free  Air 

Sec 

per  Min. 

50 

100 

ISO 

200 

300 

500 

1000 

12.72 

25 

•  245 

•  4944 

•  735 

.98 

1.47 

2-45 

4.9 

25-44 

50 

.q8l 

1.962 

2-943 

3.924 

5.886 

9.81 

19.62 

38.16 

75 

2.23 

4-45 

6.68 

8.9 

13-35 

50.83 

IOO 

3.925 

7.85 

11.77 

15-7 

ij-inch  Pipe. 


Velocity 

Volume 
of 

Length  of  Pipe  in  Feet. 

Feet  per 
Sec. 

Free  Air 
per  Min. 

50 

IOO 

ISO 

200 

300 

500 

1000 

6.7 

25 

.0567 

•"34 

.1701 

.2268 

.3402 

.567 

I  -134 

13-4 

50 

.2268 

•4536 

.6804 

.9072 

1.3608 

2.268 

4.536 

26.8 

IOO 

.9072 

1.8144 

2.7216 

3.6288 

5-4432 

9.072 

18.144 

40.2 

150 

2.0412 

4.0824 

6.1236 

8.1648 

12.2472 

20.412 

i^-inch  Pipe. 


Velocity 
in 

Volume 
of 

Length  of  Pipe  in  Feet. 

Feet  per 

Free  Air 

Se<T 

per  Min. 

50 

IOO 

ISO 

200 

300 

500 

1000 

4-9 

25 

.0172 

•0344 

.0516 

.0688 

.1032 

.172 

•344 

9.8 

50 

.0688 

.1376 

.2064 

•2752 

.4128 

.688 

1.376 

19.6 

IOO 

.2752 

•5504 

.8256 

I.  1008 

1.6512 

2.752 

5-504 

29-4 

ISO 

.6192 

1.238411.8576 

2.4/68 

3.7I52 

6.192 

12.384 

39-2 

200 

I.I008 

2.201613.30244.4032 

6.6048 

11.008 

22.016 

122 


COMPRESSED   AIR. 

TABLE    IX.— (Continued.) 

2-inch  Pipe. 


Velocity 

Volume 

Length  of  Pipe  in  Feet. 

in 

of 

Sec. 

per  Min. 

50 

100 

ISO 

200 

300 

500 

1000 

6.369 

50 

.0192 

•0384 

.0576 

.0768 

.1152 

.192 

.384 

12.738 

100 

.0768 

•  1536 

.2304 

.3072 

.4608 

.768 

1-536 

19.107 

150 

.1728 

.3456 

.5184 

.6912 

1.0368 

1.728 

3.456 

25.476 

200 

.3072 

.6144 

.9216 

1.2288 

1.8432 

3.072 

6.144 

31.845 

250 

.48 

.96 

1.44 

1.92 

2.88 

4.8 

9.6 

38.214 

300 

.6912 

1.3824 

2.0736 

2.7648 

41.472 

6.912 

13.824 

2^-inch  Pipe. 


Velocity 

Volume 

Length  of  Pipe  in  Feet. 

in 

of 

Feet  per 

Free  Air 

Sec. 

per  Min. 

100 

200 

300 

400 

500 

1000 

2000 

8.163 

100 

.0428 

.0856 

.1284 

.1712 

.214 

.428 

.856 

16.326 

200 

.1712 

•  3424 

•5136 

.6848 

.856 

1.712 

3.424 

24.489 

300 

.3859 

.7718 

I-I577 

I.5436 

1.9295 

3.859 

7.718 

32.65 

400 

.6848  1.3696 

2.0544 

2.7392 

3-424 

6.848 

13-696 

40.81 

500 

1.072 

2.144 

3.216 

4.288 

5.36 

10.72 

21.44 

3-inch  Pipe. 


Velocity 

Volume 

Length  of  Pipe  in  Feet. 

of 

Sec. 

per  Min. 

100 

200 

300 

400 

500 

IOOO 

2000 

5-659 

100 

.01647 

.03294 

.04941 

.06588 

.08235 

.1647 

•3294 

II.3I8 

200 

.06588 

.13176  .19764 

.26352 

•3294 

.6588 

1.3176 

16.977 

300 

.14823 

.29646  .44519 

.59292 

•74"5 

1.4823 

2.9646 

22.636 

400 

.26352 

.52704'.  79056 

1.054 

1.3176 

2.6352 

5-2704 

28.295 

500 

•4"75 

•8235 

1.233 

1.647 

2.058 

4.1175 

8.235 

56.59 

IOOO 

1.647 

3.294 

4.941 

6.588 

8.235 

16.47 

COMPRESSED-AIR    TRANSMISSION. 

TABLE    IX.— {Continued.} 

3i-inch  Pipe. 


I23 


Velocity 

Volume 
of 

Length  of  Pipe  in  Feet. 

Feet  per 

Free  Air 

Sec 

per  Min. 

too 

200 

300 

400 

500 

IOOO 

2000 

10.661 

250 

.04202 

.08404 

.12606 

.16808 

.2IOI 

.4202 

.8404 

21.32 

500 

.16808'.  33616 

•  50424 

.67232 

.8404 

1.68 

3-36 

31.98 
42.64 

750 
IOOO 

.37817 
.67232 

•75634 
1-344 

1.134511.5127 
2.0169  2.6893 

I.«9 
3-36 

3-78 
6.72 

7-5<> 
13.446 

53-3 

1250 

1.0505 

2.101 

3-I5I5 

4.202 

5.25 

10.505 

21.  OI 

4-inch  Pipe. 


Velocity 

Volume 
of 

Length  of  Pipe  in  Feet. 

Feet  per 

Free  Air 

Sec*! 

per  Min. 

100 

200 

300 

400 

500 

IOOO 

2000 

15   91 

500 

.08074 

.16148 

.2422 

.3229 

.4037 

.8074 

1.6,5 

23.86 

750 

.18166 

.3633 

-545 

.7266 

.908 

1.  816 

3-633 

31.82 

IOOO 

.32296 

.6459 

.969 

1.29 

1.615 

3.229 

6.459 

39-77 

1250 

.5046 

1.009 

1  .514 

2.018 

2.523 

5.046 

10.092 

47-73 

1500 

.7267 

1-4534 

2.18 

2.907 

3-633 

7.267 

'4-534 

5-inch  Pipe. 


Velocity 
-  .     in 

Volume 
of 

•    Length  of  Pipe  in  Feet. 

Feet  per 

Free  Air 

Sec 

per  Min. 

500 

IOOO 

2000 

3000 

4000 

5000 

10000 

IO.I8 

500 

.11896 

.2379 

-4758 

.7137 

•9517 

1.189 

2.379 

20.36 

IOOO 

.4758 

.9517  1.9033 

2.855 

3-8067 

4-758 

9.516 

30.54 

1500 

1.0706 

2.14134.28266.424 

8.565 

10.706 

21.41 

40.72 

200O 

I-9033 

3.8067  7-613  |"-42 

15.227 

19  033 

50.90 

25OO 

2.974 

5-948 

11.89617.844 

23-79 

29.74 

124 


COMPRESSED   AIR. 

TABLE  IX.— (Continued.) 

6-inch  Pipe. 


Velocity 

Volume 

Length  of  Pipe  in  Feet. 

of 

Feet  per 

Free  Air 

Sec1. 

per  Min. 

500 

IOOO 

2000 

3000 

4000 

5000 

10000 

14.18 

IOOO 

.1786 

.3572 

.7144 

1.0716 

1.428 

1.786 

3-572 

21.27 

1500 

.4018 

.8037 

1.6074 

2.411 

3-215 

4.018 

8.037 

28.36 

2OOO 

.7144 

1.4288 

2.857 

4.286 

5.715 

7-144 

14.288 

35-45 

2500 

I.II6 

2.232 

4-465 

6.697 

8-93 

11.162 

22.325 

42.54 

3000 

1.607 

3.215 

6.43 

9-645 

12.86 

16.075 

32.15 

8-inch  Pipe. 


Velocity 

Volume 

Length  of  Pipe  in  Feet. 

m 

Feet  per 

Free  Air 

Sec 

per  Min. 

IOOO 

2000 

4000 

6000 

8000 

10000 

15000 

I5-91 

2OOO 

.296 

•592 

1.184 

1.776 

2.368 

2.96 

4-44 

19.88 

2500 

.4626 

.925 

1.85 

2-775 

3-7 

4.62 

6-939 

23.86 

3000 

.6661 

1-332 

2.664 

3.996 

5-329 

6.66 

9-99 

31-82 

4000 

1.184 

2.368 

4-737 

7-105 

9-474 

11.842 

17.76 

39-775 

5000 

1.85 

3-701 

7.402 

11.103 

14-8 

18.505 

27-757 

10-inch  Pipe. 


Velocity 

Volume 

Length  of  Pipe  in  Feet. 

in 

of 

Feet  per 

Free  Air 

Sec. 

per  Min. 

2000 

4000 

6000 

8000 

IOOOO 

15000 

20000 

IO.I8 

2000 

.1844 

.3688 

•5532 

•7376 

.922 

I.383 

1.844 

12.73 

2500 

.288 

•5763 

.8644 

1.  1526 

1.44 

2.161 

2.88 

25-46 

5000 

i-'5 

2-3°5 

V4S8 

4.61 

5.763 

8.644 

"-5 

38.19 

7500 

2-59 

5-186 

7.78 

10.37 

12.967 

19-45 

50  92 

IOOOO 

4-61 

9.22 

13-  ^ 

18.44 

23.05 

COMPRESSED-AIR    TRANSMISSION. 

TABLE  IX.— (Continued.} 

12-inch  Pipe. 


125 


Velocity 

Volume 
of 

Length  of  Pipe  in  Feet. 

Feet  per 

Free  Air 

Sec 

per  Min. 

2000 

4000 

6000 

8000 

IOOOO 

15000 

20000 

8.84 

2500 

.11075 

.2215 

•332 

•443 

•  5537 

.83    1.1075 

17.68 

5OX> 

•443 

.886 

1.329 

1.772 

2.215 

3.3224.43 

26.52 

7500 

.9967 

1-993 

2-99 

3-987 

4-98 

7-47    9-96 

35-36 

IOOOO 

1.772 

3-544 

7.088 

8.86 

13.29 

17.72 

44.2 

12300 

2.769 

5-538 

8-3 

11.07 

13.84 

20.74 

CHAPTER   XIII. 
THE   UP-TO-DATE  AIR-COMPRESSOR. 

THE  principal  thing  to  be  said  of  the  up-to-date  air-com- 
pressor is  that  it  is  not  up  to  date.  It  would  be  difficult 
even  now,  and  notwithstanding  the  improvements  which  we 
are  told  have  been  made  in  air-compressors  in  the  last  few 
years,  to  find  the  one  that  embodies  the  best  knowledge  of 
the  time,  or  that  in  actual  performance  accomplishes  what 
should  be  expected  of  it  with  our  present  knowledge  of  the 
practical  conditions  of  economical  compression.  The 
standard  of  performance  for  a  single-stage  air  compressor 
may  be  taken  to  be  :  a  cylinderful  of  free  air  at  normal 
temperature  compressed  isothermally,  and  ail  delivered  to 
the  receiver,  by  an  apparatus  involving  no  losses  through 
friction,  and  we  should  expect  to  realize  a  nearer  approach  to 
that  standard  than  we  do.  We  should  in  the  first  place  be 
able  to  ascertain  what  is  actually  done  in  economical  air- 
compression  to-day,  and  if  any  one  undertakes  that  he  will 
find  that  it  is  no  simple  task.  The  catalogues  of  air  com- 
pressor manufacturers  are  interesting  in  this  connection, 
and  the  alleged  indicator-diagrams  contained  in  them  are 
worthy  of  study.  I  have  learned  from  them,  if  nothing 
else,  to  respect  the  wisdom  of  the  builder  who  does  not 
allow  the  diagrams  from  his  steam-  and  air-compressing 
cylinders  to  be  seen. 

While,  as  we  know,  air-compressors  are  built  and  running 
with  the  air-compressing  cylinders  placed  tandem  to  the 

126 


THE    UP-TO-DATE  AIR-COMPRESSOR.  12? 

steam-cylinders,  the  piston  rod  of  the  steam-cylinder  being 
continued  into  the  air-cylinder  and  transmitting  all  the 
power  required  for  compression  directly  to  the  compressing 
piston,  and  with  a  friction  loss  of  only  5  per  cent  between 
the  steam-cylinder  and  the  air-cylinder,  there  are  indicator- 
diagrams  published  in  builders'  catalogues  that  show  very 
different  results.  In  one  set  of  diagrams,  bearing  every 
evidence  of  genuineness,  but  published  without  data,  the 
ratio  of  the  air-cylinder  M.E.P.  to  that  of  the  steam- 
cylinder  is  .633,  a  loss  of  over  36  per  cent  in  power  alone, 
saying  nothing  of  the  other  inevitable  losses.  In  another 
catalogue  a  set  of  alleged  indicator-diagrams  is  given  with 
some  accompanying  data,  and  with  a  ratio  of  air-card  to 
steam-card  of  .818,  a  loss  of  18  per  cent.  A  diagram  from 
an  air-compressing  cylinder,  published  by  another  manu- 
facturer, shows  the  air-admission  line  above  the  atmosphere- 
line  for  almost  the  entire  length  of  it,  as  though  the  air 
would  rush  into  the  air-cylinder  with  alacrity  when  the 
pressure  was  higher  within  the  cylinder  than  outside  of  it! 
Still  another  builder,  commenting  in  his  catalogue  upon  this 
phenomenon,  says  that  the  fact  that  the  air-admission  line 
is  above  the  atmosphere-line  proves  that  his  rival's  piston 
leaks.  I  have  in  my  possession  still  another  indicator- 
diagram  from  a  compressing  cylinder  with  newly  patented 
valves,  and  in  which  the  air  pressure  in  the  cylinder  at  the 
beginning  of  the  compression-stroke  is  ten  pounds  above 
the  atmosphere,  although  the  cylinder  is  filled  with  free  air 
at  each  stroke  and  the  entire  compression  is  done  in  that 
one  cylinder.  And  so  it  goes.  We  may  say  that  the  air- 
compressor  builders  are  living  upon  the  ignorance  of  their 
customers,  or  we  may  say  that  the  blind  are  leading  the 
blind,  as  may  seem  most  correct  for  the  individual  case. 


128  COMPRESSED   AIR, 

Of  all  the  steam-actuated  air-compressors  in  existence 
the  one  showing  the  very  worst  results,  as  far  as  economy 
of  steam  for  the  service  performed  is  eoncerned,  is  the  air- 
compressor  used  upon  locomotives  for  operating  the  air- 
brakes. To  compress  a  given  volume  of  free  air  to  a  cer- 
tain pressure  the  "  air-brake  pump  "  uses'  nearly  ten  times 
as  much  steam  as  would  be  required  in  the  best  air-com- 
pressors of  the  day  for  the  same  service.  The  air-brake 
pump,  however,  is  the  one  compressor  whose  extravagant 
waste  of  steam  is  condoned  by  the  circumstances  surround- 
ing its  employment.  There  are  more  than  30,000  of  these 
pumps  in  use,  a  number  greater,  perhaps,  than  that  of  all 
other  air- compressors  combined,  not  counting  those  that 
are  used  for  beer.  While  thd  wastefulness  of  this  pump  is 
fully  conceded,  its  persistent  use  for  air-brake  service  is 
completely  vindicated.  The  pump  is  very  simple  and 
always  ready,  which  is  an  important  point,  and  the  steam 
used  to  operate  it  upon  the  locomotive  is  mostly  steam  that 
otherwise  would  be  blown  off  by  the  pop  safety-valve. 
The  pump  is  usually  worked  when  stops  are  made  at 
stations  or  when  running  down  grade,  and  if  the  pump 
used  much  less  steam  it  would  generally  mean  not  that  so 
much  steam  was  saved,  but  that  the  safety-valve  would 
have  so  much  more  to  do.  Various  styles  of  air-brake 
pumps  have  been  devised  showing  a  much  better  economy, 
but  they  have  been  successively  abandoned  for  the  estab- 
lished pump.  It  is  only  when  the  air-brake  pump  is  used 
for  the  purpose  of  a  general  compressed-air  supply,  as  it 
quite  frequently  is  in  railroad  shops,  that  its  extravagance 
is  to  be  condemned.  In  such  cases  no  language  can  be  too 
severe  to  characterize  the  folly  of  it.  That  the  air-brake 
pump  can  be  used  with  profit  and  satisfaction  to  supply 


THE    UP-TO-DATE  AIR-COMPRESSOR. 


I29 


compressed  air  for  general  use  speaks  highly  of  the  value  of 
the  air. 

As  a  mechanical  curiosity,  and  as  exhibiting  a  great 
achievement  of  ingenuity,  a  set  of  indicator-diagrams  from 
an  air-brake  pump  are  here  reproduced,  Fig.  20  being  from 


Fig.  2O. 

the  steam-cylinder  and  Fig.  21  from  the  air-cylinder.  The 
steam-cylinder  diagrams  are  so  different  from  the  familiar 
cards  of  the  ordinary  steam-engine  cylinder  that  it  has  been 
thought  best  to  place  the  arrows  upon  them  to  indicate  the 
direction  of  motion.  They  would  look  more  natural  to  the 


Fig.  21. 


general  steam  engineer  if  he  could  be  allowed  to  read  thei 
in  the  reverse  way.  It  will  be  noticed  that  the  steam 
pressure  in  the  cylinder  is  low  at  the  beginning  of  the 
stroke,  corresponding  with  the  low  resistance  in  the  air- 
cylinder,  and  that  the  steam  pressure  rises  with  the  progress 
of  the  stroke,  and  at  the  end  of  it  the  cylinder  is  full  of 


I3O  COMPRESSED   AIR. 

high-pressure  steam,  while  that  steam  has  done  much  less 
work  than  would  be  due  to  the  dead  pressure  of  that  volume 
of  steam,  saying  nothing  of  the  additional  power  that  could 
have  been  developed  by  using  expansively.  This  is  evi- 
dently a  more  wasteful  application  of  steam  even  than  in  the 
direct-acting  pump  for  water.  This  distribution  of  steam, 
however,  accomplishes  the  designed  purpose  of  approxi- 
mately equalizing  the  steam  pressure  to  the  resistance,  and 
the  air-brake  pump  is  thus  enabled  to  dispense  with  the 
crank-shaft  and  all  which  it  implies. 

Under  any  arrangement  that  has  been  invented  for  using 
steam  economically  the  pressure  in  the  steam-cylinder 
during  the  earlier  part  of  the  stroke  is  at  its  highest,  and 
decreases  generally  to  nothing,  or  nearly  nothing,  at  the  end 
of  the  stroke.  In  opposition  to  this  the  resistance  in  the 
air-cylinder  at  the  beginning  of  the  compression-stroke  is 
very  low  and  increases  as  the  piston  advances,  and  at  the 
latter  part  of  the  compression-stroke  this  resistance  is  con- 
siderably higher  than  the  force  of  the  steam  that  is  driving 
the  piston.  To  keep  the  compressor  in  motion  it  is  not 
enough  that  the  mean  effective  pressure  upon  the  steam- 
piston  for  the  whole  stroke  shall  exceed  the  mean  effective 
resistance  against  the  air-piston  plus  the  friction  of  the 
entire  apparatus.  The  force  and  resistance  must  be  equal- 
ized in  some  way  to  keep  up  the  movement,  and  various 
devices  have  been  employed  for  this  purpose.  The  usual 
reliance  at  the  present  time  is  upon  the  weight  of  the 
reciprocating  parts  and  heavy  fly-wheels,  and  it  is  doubtful 
still  if  there  is  anything  that  is  in  all  respects  to  be  preferred. 
A  novel  and  ingenious  arrangement  for  accomplishing  this 
desired  object  has  lately  been  brought  out  by  one  of  the 
air-brake  companies,  not  so  much,  it  is  understood,  for  air- 


THE    UP-TO-DATE  AIR-COMPRESSOR.  13 1 

brake  service,  as  for  general  use  in  air-compression.  The 
two  air-cylinders  of  this  compressor  are  horizontal  and 
single-acting,  and  they  together  form  the  foundation  for 
the  entire  compressor.  While  they  are  together  equal  in 
free  air  capacity  to  a  double-acting  cylinder  of  the  same 
diameter  and  stroke,  they  are  in  other  respects  quite  differ- 
ent, as  the  pistons  have  movements  independent  of  and 
always  different  in  speed  from  each  other,  except  momen- 
tarily at  a  point  near  the  middle  of  each  stroke.  Above  the 
air  cylinders  is  placed  the  steam-engine,  which  forms  a  part 
of  and  which  actuates  the  air-compressor.  The  engine 
comprises  the  usual  elements  of  the  horizontal  steam- 
engine — the  steam-cylinder  and  its  piston,  the  cross-head, 
connecting-rod,  crank-shafts,  fly-wheels,  and  the  mechanism 
of  the  valve  motion.  Short  connecting-rods  attached  to 
the  cross-head  give  motion  to  two  compensating  levers  with 
changing  fulcrums,  and  through  these  levers  power  is  trans- 
mitted to  the  air-compressing  pistons;  and  with  a  uniform 
movement  assumed  for  the  cross-head  a  continually  decreas- 
ing movement  is  given  to  each  air-piston  for  its  compres- 
sion-stroke. At  the  beginning  of  either  stroke  of  the  steam- 
piston  the  fulcrum  of  the  equalizing  lever  is  above  the 
middle  of  it,  and  the  air-piston  moves  faster  than  the  steam- 
piston.  At  the  latter  part  of  the  stroke  of  the  steam-piston 
the  fulcrum  of  the  lever  is  nearer  its  lower  end,  and  the  air- 
piston  then  moves  much  slower  than  the  steam-piston. 
The  indicator-diagrams  Fig.  22  show  the  practical  opera- 
tion of  this  compressor.  The  upper  diagram,  from  the 
steam-cylinder,  shows  the  steam  at  100  pounds  cut-off  at 
four  tenths  of  the  stroke.  The  dotted  line  of  the  diagram 
shows  the  effect  of  the  steam  pressure  for  the  stroke  as 
modified  by  the  weight  and  inertia  of  the  reciprocating 


I32  COMPRESSED    AIR. 

parts.     The  lower  diagram,  from  the  air-cylinder,  exhibits 


Air  Cylinder*  lOfe 
Fig.  22. 


the  operation  of  compressing  free  air  up  to  and  delivering 


THE    UP-TO-DATE  AIR-COMPRESSOR.  133 

it  at  100  pounds  pressure.  The  dotted  lines  in  this  diagram 
show  the  resultant  force  from  the  steam-piston  as  trans- 
mitted by  the  action  of  the  compensating  lever  to  the  air- 
piston.  It  is  evident  that  the  work  required  of  the  fly-wheel 
in  this  case  is  less  than  in  the  ordinary  steam-engine,  while 
in  the  common  air-compressor  it  is  much  greater.  These 
cards  show  the  friction  of  the  compressor  to  be  high,  the 
ratio  of  the  air  to  the  steam-cylinder  diagram  being  .75,  a 
loss  of  25  per  cent  from  this  source  alone. 

The  full  sponsorial  and  patronymic  appellation  of  the 
most  pretentious  member  of  the  air-compressor  family 
to-day  is  the  Corliss  Cross-Compound  Condensing  Com- 
pressor. It  may  be  called  the  Five  C's.  The  "  cross  "  is 
not  practically  as  good  as  the  tandem,  but  commercially 
the  alliterative  effect  is  valuable.  The  Corliss  feature  is 
one  of  the  most  valuable  adjuncts  for  selling  the  com- 
pressor, but  has  nothing  to  do  with  operating  it.  The 
Corliss  engine,  as  everybody  knows,  is  designed  to  main- 
tain a  uniform  speed  under  a  varying  load.  The  cut-off^ 
controlled  by  the  governor,  is  changed  as  the  load  changes 
and  because  the  load  changes.  The  air-compressor  is 
required  to  maintain  a  constant  air  pressure  when  there  is 
a  varying  demand  for  the  air,  and  this  varying  demand 
means  of  course,  and  can  only  mean,  a  varying  speed  of 
operation,  so  that  to  take  a  fully  equipped  Corliss  stationary 
engine  and  to  attach  an  air-cylinder  tandem  to  the  steam- 
cylinder,  or,  if  a  double  or  compound  engine,  to  attach  an 
air-cylinder  tandem  to  each  steam-cylinder,  the  propriety  of 
the  arrangement  must  be  very  evident  to  those  who  can  see 
it.  All  computations  upon  the  efficiencies  of  air-com- 
pressors have  been  based  upon  the  assumption  of  constant 
work  under  the  best  conditions.  When  it  is  recognized 


134  COMPRESSED   AIR. 

that  no  compressed-air  service  is  uniform  in  its  demands, 
then  the  sacrifice  of  ideal  conditions  that  the  varying 
demand  entails  becomes  quite  an  important  factor  in 
determining  the  ultimate  economy  of  the  system.  How  a 
compressor  is  governed  is  a  very  pertinent  question  for  the 
economist.  I  cannot  afford  to  go  into  it  here,  but  I  may 
say  that  nine  tenths  of  all  the  air-compressors  in  use,  not 
including  the  air-brake  pumps,  have  no  governors,  and  the 
governing  devices  employed  upon  most  of  the  others  are 
crude,  unsatisfactory,  and  generally  disgraceful. 

Where  large  air-compressing  plants  are  to  be  established 
for  continuous  service,  a  much  higher  ultimate  economy  can 
be  attained  than  where  the  plant  required  is  not  so  extensive. 
It  is  best  to  use  a  number  of  units  for  the  work  of  compres- 
sion instead  of  one  or  two  large  compressors.  Air-com- 
pression offers  little  or  no  opportunity  for  the  storage  of 
power  or  for  doing  any  work  in  advance,  as  may  be  done  by 
a  water-pump  and  reservoir.  The  receivers  used  in  con- 
nection with  air-compressors  will  not  usually  hold  more 
than  the  compressor  can  deliver  in  one  minute,  so  that  if 
the  demand  for  the  air  fluctuates  it  must  be  met  by  the 
speed  of  delivery  at  the  compressors,  and  not  by  a  change 
of  reservoir  supply.  Air-compressors,  like  simple  steam- 
engines,  have  their  conditions  of  speed,  pressure,  etc.,  that 
secure  the  best  economy;  and  where  a  plant  consists  of  a 
number  of  units,  all  in  operation,  it  will  usually  be  more 
economical  to  let  most  of  them,  or  as  many  as  possible,  run 
steadily  at  their  best,  and  to  do  the  governing  or  equalizing 
of  the  work  by  one  or  two  of  the  compressors  rather  than  by 
all  of  them.  The  more  extensive  the  air-compressing  plant 
may  be  or  the  more  extensive  the  use  of  the  air  compressed 
the  more  uniform  the  demand  may  be  expected  to  be. 


CHAPTER  XIV. 

COMPRESSED   AIR    VERSUS  ELECTRICITY. 

THE  title  of  this  chapter  is  adopted  in  deference  to  the 
prevalent  idea  of  the  relations  of  these  two  power-trans- 
mitters. To  my  thinking  the  versus  should  be  read  as 
lightly  as  it  is  possible  to  read  it,  for  there  is  in  fact  but 
little  antagonism  or  competition  between  compressed  air 
and  electricity,  and  there  is  little  likelihood  that  in  practice 
there  ever  will  be.  Neither  of  them  is  a  power-transmitter 
pure  and  simple,  as  a  wire  rope  may  be  said  to  be,  but  each 
is  capable  of  performing  other  functions,  and  the  power- 
transmitting  capabilities  of  each,  in  combination  with  their 
other  individually  peculiar  lines  of  usefulness,  open  foi 
each  a  distinct  and  separate  field,  which  neither  can  fill  foi 
the  other.  The  same  is  true  of  some  of  the  other  power- 
transmitters.  They  each  have  their  special  fields  of  useful- 
ness and  adaptability  which  neither  of  the  others  could  fill 
as  well,  if  at  all. 

Of  late  the  gas-engine  has  been  coming  rapidly  to  the 
front  as  a  valuable  agent  in  the  development,  transmission, 
and  distribution  of  power,  and  it  has  its  enthusiastic  advo- 
cates who  are  ready  to  predict  that  before-  long  it  is  to 
supersede  every  other  motor  over  a  field  that  is  practically 
boundless.  But  upon  looking  over  the  field  a  little  farther 
and  listening  to  another  group  of  enthusiasts  it  soon  appears 
that  not  the  gas-engine  but  the  oil-engine  is  the  coming 

i35 


136  COMPRESSED   AIR. 

motor,  and  not  only  is  it  the  coming  motor,  but  it  has 
already  come,  and  is  driving  out  the  electric  and  gas  and 
other  motors,  and  the  steam-engine  also,  in  England  and 
Germany  and  elsewhere  in  Europe,  and  it  must  soon  do  so 
also  with  us  in  the  United  States.  But  as  we  look  into  the 
operating  conditions  under  which  these  several  agencies 
may  find  employment  we  soon  learn  that  each  of  the  several 
motors  is  most  applicable  under  conditions  of  its  own,  and 
that  neither  can  do  all  that  either  of  the  others  can  do. 
Gas  and  oil,  of  course,  develop  power  as  the  steam  engine 
does,  while  compressed  air  and  electricity  can  only  trans- 
mit power  that  originates  elsewhere.  But  with  the  devel- 
opment and  transmission  of  power  the  usefulness  of  gas 
("  producer "  gas)  or  of  oil  ends,  while  with  air  and  elec- 
tricity power-transmission  is  not  their  only  function. 
Electricity  has  the  vast  field  of  illumination,  in  which  it 
reigns  supreme  ;  compressed  air  has  no  one  application  to 
compare  in  magnitude  and  importance  with  that  of  electric 
lighting,  but  it  has  a  vast  number  of  duties  which  are  all  its 
own,  and  which  electricity  cannot  touch.  The  use  of  com- 
pressed air  has  been  slow  of  development,  and  is  still  back- 
ward, but  at  this  writing  I  am  able  to  enumerate  two 
hundred  distinct  and  established  uses  of  compressed  air, 
and  in  more  than  90  per  cent  of  those  uses  electricity  is 
absolutely  inapplicable,  and  in  the  remainder,  which  form 
a  field  more  or  less  open  to  other  agencies  besides  either  air 
or  electricity,  the  air  generally  has  the  advantage.  Turn  to 
the  last  chapter  of  this  little  book,  wherein  some  of  the 
uses  of  compressed  air  are  enumerated,  and  see  all  those 
that  come  under  the  first  letter  of  the  alphabet  and  judge 
where  the  competition  with  electricity  comes  in.  In  our 
list  of  the  applications  of  compressed  air  some  of  the  other 


COMPRESSED    AIR    VERSUS  ELECTRICITY.     137 

letters  of  the  alphabet  develop  a  larger  enumeration  than 
the  first  letter,  and  the  use  of  air  for  operating  motors,  or 
for  producing  rotary  motion  in  general,  or  for  performing 
any  of  the  functions  of  the  steam-engine,  is  not  included. 
Referring  to  the  portion  of  the  list  under  the  letter  A,  it  will 
be  noticed  that  the  only  applications  of  air  that  compete 
with  electricity  are  the  air-brake  and  the  air-hoist  or  the  air- 
jack.  The  electric  brake  in  competition  with  the  air-brake 
is  anything  but  a  success,  and  it  is  not  worth  further  men- 
tion. Even  upon  electric  cars  the  air-brake  is  an  absolute 
necessity  for  safety,  and  hundreds  of  lives  have  been  sacri- 
ficed in  our  city  streets  because  it  has  not  been  used.  The 
air-jack  also  has  the  field  to  itself,  and  electricity  is  "not 
in  it."  In  the  field  of  general  hoisting  air  and  electricity 
divide  the  work,  and  the  line  of  service  done  by  each  is  gen- 
erally distinct  from  that  performed  by  the  other.  There 
are  establishments  where  they  are  thoroughly  familiar  with 
the  uses  and  capabilities  of  electricity,  operating,  for  in- 
stance, electric  travelling  cranes,  and  yet  which  use  com- 
pressed air  in  numerous  places  throughout  their  works  for 
hoisting,  and  where  for  the  special  services  required  electric- 
ity would  have  no  chance  at  all.  Where  the  direct-acting 
vertical  hoist  can  be  used,  or  the  air-cylinder,  either  verti- 
cal or  horizontal,  with  multiplying  sheaves  and  a  wire  rope, 
it  is,  of  course,  preferable  to  electricity  with  its  spinning 
motor-shaft,  its  drums  and  gearing.  In  the  general  work  of 
hoisting  as  carried  on  at  the  Armour  Packing  Company's 
vast  establishment  electricity  could  not  possibly  do  the  work 
that  the  air  does.  The  wonderful  capability  of  standing 
ready  for  instant  use  at  full  power  and  without  cost  for 
maintenance  for  long  periods  of  time  seems  to  be  possessed 
by  compressed  air  alone.  It  is  pre-eminently  adapted  to 


138  COMPRESSED    AIR. 

uses  that  call  for  constant  alertness,  as  in  the  switch  and 
signal  service  and  in  the  air-brake,  and  in  the  air-hoist  it 
stands  at  its  post  day  and  night  ready  to  give  a  lift  the 
instant  it  is  called  upon. 

In  the  lines  of  service  to  which  electricity  and  com- 
pressed air  seem  to  be,  perhaps,  equally  applicable,  and 
where  they  could  compete  with  no  apparent  disadvantage 
to  either,  it  is  to  be  regretted  that  circumstances  seem 
invariably  to  defeat  a  fair  comparison.  In  driving  pumps 
a  very  fair  test  could  be  instituted  of  the  relative  merits  of 
each,  and  of  the  losses  in  the  use  of  each,  as  power-trans- 
mitters, and  it  happens  that  in  this  very  work  of  pumping 
we  find  some  striking  illustrations  of  what  might  be  termed 
the  constant  bad  luck  accompanying  the  air,  or  the  malig- 
nant opposition  of  circumstances  to  any  fair  exhibition  of 
its  powers.  Compressed  air,  by  its  very  accommodating 
attitude,  by  its  very  applicability  to  widely  varying  con- 
ditions, is  constantly  placing  itself  in  a  false  position  before 
the  community  and  showing  itself  at  a  disadvantage.  It  is 
able  to  accept  conditions  that  enable  it  to  make  an  un- 
seemly and  unjust  exhibition  of  its  powers,  yet  which 
entirely  exclude  electricity  from  any  such  depreciatory 
performance.  The  pump  that  can  be  operated  by  electric- 
ity can  be  operated  equally  well  by  compressed  air,  the  air- 
motor  taking  the  place  of  the  electric  motor,  either  of  them 
producing  rotary  motion ;  and  with  suitable  connecting 
gearing  and  with  the  pump  mechanism  unchanged  one  would 
have  as  good  a  chance  as  the  other,  and  under  those  con- 
ditions the  air  could  be  made  to  do  better  than  the  elec- 
tricity. 

Electricity  seems  to  be  making  advances  more  rapid  than 
ever  before  in  its  employment  for  railway  traction.  It 


COMPRESSED    AIR    VERSUS   ELECTRICITY.      139 

drives  the  horses  from  the  surface  roads,  and  is  now  be- 
ginning to  supersede  the  steam  locomotive.  Perhaps  all  do 
not  realize  that  this  is  the  triumph  after  all  of  the  steam 
engineer  more  than  of  the  electrical  engineer.  Electricity  is 
demonstrating  not  so  much  its  superiority  ars  a  power-trans- 
mitter, but  is  simply  showing  the  ultimate  economy  of  gener- 
ating power  in  large  central  plants,  even  if  the  means  of  dis- 
tribution is  a  wasteful  one,  and  accompanied  by  features  that 
are  insurmountably  objectionable.  The  extending  use  of 
electricity  as  a. railway  motor  is  an  argument  also  for  com- 
pressed air,  for  it  is  able  to  take  full  advantage  of  the 
economy  in  centralized  power  development,  and  we  are  in 
the  way  to  see  very  soon  some  practical  demonstration  of 
its  abilities  in  this  field.  In  railroad  service,  as  in  every- 
thing else,  compressed  air  has  been  heretofore  unfortunate, 
and  its  advocates  and  would-be  promoters  have  wasted 
time  and  opportunity  in  developing  minute  economies  in 
the  air-motor  which  were  not  needed  to  enable  it  to  com- 
pete with  the  best  in  the  field.  It  may  be  regarded  as  cer- 
tain that  whatever  gain  may  be  shown  in  the  employment 
of  electricity  for  traction  its  establishment  is  by  no  means 
a  final  solution  of  any  question  except  of  the  economical 
generation  of  power,  and  that  electricity  has  nothing  to  do 
with. 

Has  any  one  called  attention  to  the  fact  that  one  of  our 
most  prominent  and  perplexing  political  questions  is  entirely 
and  indisputably  the  product  of  compressed  air?  Can 
electricity  claim  to  have  contributed  any  prominent  factor 
in  determining  the  course  of  parties  or  in  shaping  the 
destinies  of  the  nation  ?  What  if  compressed  air  should  be 
Tound  to  hold  the  balance  of  power  and  the  deciding  voice 
in  the  selection  of  a  future  President  of  the  United  States  ? 


I4O  COMPRESSED    AIR. 

This  is  the  actual  situation,  and  not  an  absurd  or  exag- 
gerated statement  of  it.  The  only  political  function 
attained  by  electricity  is  that  of  public  executioner.  Elec- 
tricity and  compressed  air  stand  to  each  other  as  the 
masked  and  nameless  headsman  upon  the  one  side  and 
Warwick  the  King-maker  upon  the  other.  The  silver  ques- 
tion of  the  day,  whichever  side  of  it  we  may  find  ourselves 
on,  is  entirely  the  outgrowth  of  the  increased  output  of 
silver,  and  that,  no  one  can  deny,  is  what  the  air-driven 
rock  drill  has  accomplished.  The  precipitation  upon  us  of 
this  perplexing  question  may  have  been  a  work  of  question- 
able beneficence,  but  the  power  that  could  achieve  it  is  not 
to  be  treated  lightly. 


CHAPTER  XV. 
THE  THERMAL  RELATIONS  OF  AIR  AND  OF  WATER. 

IN  all  of  our  operations  with  compressed  air,  either  in 
its  compression,  its  storage  and  transmission,  or  in  its  final 
application  to  whatever  purpose,  the  temperature  of  the  air 
at  any  time,  and  the  effect  of  raising  or  lowering  its  temper- 
ature by  whatever  means,  are  always  important  facts  to  be 
considered,  and  it  will  be  well  for  us  as  early  as  possible  to 
fix  in  our  minds  some  general  ideas  upon  the  subject.  The 
thermal  relations  of  water  are  so  different  from  those  of 
air  that  by  contrast  a  knowledge  of  the  one  may  be  made 
to  enforce  our  knowledge  of  the  other.  The  fact  also  that 
the  effects  of  heat  upon  water  are  accepted  as  standards 
of  heat  measurements  makes  it  necessary  for  us  to  know 
something  about  them. 

Say  that  we  apply  a  given  quantity  or  unit  of  heat  to  a 
pound  of  water,  raising  its  temperature  i  degree  ;  how 
much  air  would  be  equally  heated,  or  have  its  temperature 
raised  i  degree,  by  the  same  unit  of  heat  ?  A  cubic  foot  of 
air  at  atmospheric  pressure — "  free  air  " — and  at  62  degrees 
weighs  .076  pound,  and  a  pound  of  air  therefore  in  vol- 
ume equals  i  -r-  .076  =  13.158  cubic  feet.  A  pound  of 
water  is  27.7  cubic  inches,  and  the  ratio  of  volumes  of 
water  and  of  air  of  equal  weight  will  be  about  i  :  821. 
But,  pound  for  pound,  it  takes  less  heat  to  raise  the  tem- 

141 


142 


COMPRESSED    AIR. 


perature  of  air  i  degree,  or  any  number  of  degrees,  than  is 
required  to  raise  the  temperature  of  water  the  same  num- 
ber of  degrees.  The  specific  heat  of  water  being  i,  that  of 
air  is  only  .2377,  so  that  13.158  cubic  feet  -=-  .2377  —  55 
cubic  feet,  and  this  55  cubic  feet  of  free  air  is  to  be  com- 
pared with  i  pound,  or  27.7  cubic  inches,  of  water. 

There  is  a  means  of  fixing  the  thermal  relations  of  air 
and  water  in  the  mind's  eye  so  that  they  may  not  be  easily 
forgotten.  A  common-sized  glass  tumbler,  not  quite  full, 
holds  a  half-pound  of  water.  A  cubical  box  measuring  3 
feet  each  way,  or  say  a  large 
dry  -  goods  box,  holds,  of 
course,  a  cubic  yard,  or  27 
cubic  feet,  which  is,  nearly 
enough,  a  half  of  our  55  cubic 
feet,  so  that  the  dry-goods  box 
full  of  air  and  the  tumbler 
full  of  water  represent  quite 
closely  the  volumes  of  air  and 
of  water  that  will  be  equally 
heated  by  equal  units  of  heat. 
The  approximate  ratio  of  vol- 
umes will  be  i  :  3431,  and  the  ratio  of  the  sides  of  two 
cubes  representing  the  two  volumes  will  be  i  :  15  +.  The 
isometric  projection  of  the  two  cubes  here  shown  (Fig.  23^1 
may  convey  and  impress  the  relations  better  than  the 
figures  can  do  it. 

It  is  to  be  remembered  that  in  the  transmission  of  heat 
either  to  or  from  air  or  water — that  is,  whether  heating  or 
cooling  them,  or  whether  cooling  or  heating  any  bodies  in 
thermal  communication  with  them — the  above  ratios  will 
prevail.  Those  whose  attention  is  called  for  the  first  time 


Fig.  23. 


THERMAL   RELATIONS   OF  AIR  AND    WATER.    143 

to  the  phenomena  accompanying  air-compression  or  expan- 
sion cannot  fail  to  be  struck  with  the  great  changes  of 
temperature  that  ensue  coincidently  with  either  operation, 
but  the  actual  heat  represented  by  these  changes  is  usually 
overestimated,  although  circumstances  that  should  check 
the  exaggerated  estimate  are  also  at  hand.  If  the  body  of 
the  air-compressing  cylinder  and  the  cylinder-heads  are 
properly  water-jacketed,  the  temperature  of  the  air  deliv- 
ered is  considerably-  lower  than  it  would  be  if  there  were 
no  water-jacketing,  but,  at  the  same  time,  the  perceptible 
heating  of  the  water  in  the  jacket,  by  which  the  partial 
cooling  of  the  air  is  effected,  proceeds  quite  slowly,  show- 
ing that  the  actual  quantity  of  heat  abstracted  from  the  air 
by  that  means  is  not  great.  So  also  when  the  heated  com- 
pressed air  flows  through  pipes  for  some  distance,  the  rapid- 
ity with  which  its  temperature  approaches  that  of  its  envi- 
ronment is  another  evidence  of  the  small  amount  of  heat 
actually  carried  by  it.  Still  we  hear  constantly  of  the  won- 
ders of  heating  or  cooling  that  are  to  be  done  by  compressed 
air — wonders  that  never  fully  materialize  with  a  more  ex- 
tended experience.  In  the  Pohle  "  air  lift  pump  "  (  which 
is  not  properly  a  pump,  as  it  has  absolutely  no  moving  or 
working  or  wearing  parts,  but  which  is  a  very  valuable  ap- 
plication of  compressed  air  direct  for  raising  water  from 
bored  wells,  and  where  the  air  is  discharged  upward  into  the 
submerged  end  of  a  vertical  water-pipe,  the  air  entering  the 
pipe,  distributing  itself  through  the  water,  and  rising  with  it, 
expanding  as  it  rises)  it  is  claimed  that  the  expansion  of 
the  air  while  in  contact  with  the  water  cools  the  water. 
We  may  say  that  the  expanding  air  certainly  does  cool  the 
water,  and  we  may  also  say  that  it  certainly  does  not  cool 
the  water  more  than  a  fraction  of  a  single  degree. 


144  COMPRESSED   AIR. 

Where  an  establishment  is  equipped  with  a  permanent 
compressed-air  plant,  for  operating  hoists,  jacks,  presses, 
and  isolated  machines  of  all  kinds,  it.  is  a  simple  matter  to 
rig  up  an  arrangement  for  cooling  the  drinking-water  for  the 
employes.  Take  a  i£"  pipe  100  feet  long  (50  feet  might 
be  long  enough),  place  it  horizontally,  and  connect  one  end 
of  it  to  the  compressed-air  supply,  with  a  suitable  cock  to 
control  the  escape  of  the  air.  Leave  the  other  end  of  the 
pipe  open  and  enclose  the  whole  of  the  pipe,  after  passing 
the  air-admission  cock,  in  a  thick  non-conducting  covering. 
If  nothing  better  is  at  hand,  take  plenty  of  paper,  and  wind 
it  on  spirally  layer  after  layer,  covering  the  whole  pipe. 
Then  lead  a  f "  water-pipe  into  the  open  end  of  this  hori- 
zontal released  air-pipe,  and  let  it  come  out  by  a  tee  or 
otherwise  at  the  other  end  of  the  air-pipe,  and  the  whole 
apparatus  is  provided.  The  air  in  this  case  should  be  thor- 
oughly cooled  and  have  all  its  suspended  water  discharged 
before  its  release  in  the  cooling-pipe. 

A  touching  spectacle  in  all  our  large  cities  are  those  mel- 
ancholy monuments  of  a  futile  philanthropy,  its  public 
drinking-fountains.  The  motive  that  prompts  their  erec- 
tion is  worthy  of  all  respect.  Much  money  has  been  ex- 
pended upon  them  with  the  best  of  intentions,  but  with  the 
poorest  of  results.  They  can  nowhere  be  said  to  be  a  suc- 
cess, for  they  do  not  accomplish  what  they  propose  to  do. 
They  offer  the  cup  to  the  thirsty  lip,  but  it  is  practically 
an  empty  cup,  for  it  does  not  hold  what  we  want.  Who 
wants  to  drink  warm  water  ?  The  most  costly  and  artistic 
of  fountains  is  nowhere,  in  the  thought  of  the  hot  and 
thirsty  crowd,  in  competition  with  a  bucket  of  cold  water 
and  an  old,  rusty  tin  dipper.  The  instinctive  call  of  hu- 
manity for  cold  water  to  drink  is  so  absolutely  universal 


THERMAL   RELATIONS  OF  AIR  AND    WATER.    1 45 

that  it  must  be  correct,  and  should  be  more  adequately 
provided  for. 

Our  cities  are  constantly  doing  more  and  more  for  the 
comfort  and  well-being  of  all  the  people.  To  all  of  us  life 
is  more  worth  the  living  by  reason  of  our  co-operation  and 
our  collective  helpfulness.  Amid  all  that  is  designed  to 
make  our  cities  more  attractive  and  more  desirable  to  live 
in,  is  there  any  possible  material  thing  that  can  be  sug- 
gested more  to  be  desired,  more  proper  to  do,  more  promis- 
ing of  universal  good,  than  to  make  it  possible  for  every 
man,  woman,  and  child  to  have  always  at  hand  a  drink  of 
cold  water  ?  Does  not  compressed  air  make  it  possible  ? 
The  establishment  of  a  general  compressed  air  service  in 
any  city  might  be  gloriously  celebrated  by  the  establish- 
ment of  a  cold-water  fountain. 

If  anyone  does  undertake  to  cool  drinking-water  by  the 
use  of  compressed  air,  we  may  expect  to  hear  that  it  takes 
a  great  quantity  of  air  to  cool  a  little  water,  which  is  just 
about  what  I  have  been  writing  above.  The  case,  how- 
ever, in  the  matter  of  cooling  by  compressed  air  is  not 
nearly  as  bad  as  I  seem  to  make  it  appear.  The  actual 
heat  represented  by  any  change  of  temperature  in  air  com- 
pressed to  a  pressure  of  several  atmospheres  is  of  course 
greater  than  for  air  compressed  to  only  i  atmosphere,  or 
free  air,  as  we  call  it,  and  directly  in  proportion  to  the  re- 
spective absolute  pressures.  With  air  at  75  Ibs.  gauge,  or 
6  atmospheres,  the  same  change  of  temperature  in  either 
heating  or  cooling  would  indicate  a  transfer  of  six  times  the 
amount  of  heat  that  would  be  indicated  by  the  same  heat- 
ing or  cooling  of  free  air.  The  sudden  release  of  air  com- 
pressed to  6  atmospheres  and  at  62°  before  the  release 
would  cause  a  theoretical  fall  of  temperature  of  over  200°, 


146  COMPRESSED    AIR. 

and  if  this  air  were  in  communication  with  water  that  re- 
quired to  be  cooled  but  20°,  this  would  give  the  air  a  con- 
siderable advantage.  In  the  case  of  the  Pohle  air  lift 
pump,  cited  above,  where  the  volume  of  water  would  prob- 
ably be  as  great  as  that  of  the  compressed  air  in  contact 
with  it,  the  cooling  effect  of  the  expanding  air  could  be  bur 
slight,  as  before  stated. 


CHAPTER  XVI. 
THE  FREEZING  UP  OF  COMPRESSED  AIR. 

THE  most  familiar  and  the  most  constantly  reiterated 
objection  to  the  use  of  compressed  air  is  its  well-known 
habit  of  "  freezing  up  "  under  certain  conditions,  and  too 
many  who  have  not  sufficiently  investigated  the  subject 
have  regarded  this  freezing-up  of  the  air  as  an  insurmount- 
able and  fatal  objection  to  its  use  for  purposes  for  which  it 
would  seem  to  be  otherwise  eminently  adapted.  By  "  the 
freezing  up  of  the  air,"  as  the  expression  is  commonly  used, 
— although,  of  course,  it  is  never  the  air  that  freezes, — we 
understand  a  deposition  of  moisture,  more  or  less  rapid, 
upon  the  sides  of  the  pipes  or  passages  that  convey  the  air, 
and  its  accumulating  and  freezing  there  until  the  area  of 
the  channel  is  materially  reduced,  the  proper  flow  of  the 
air  prevented,  and  the  operation  of  the  air-motor  or  other 
apparatus  seriously  impeded  or  stopped  entirely.  This 
phenomenon  may  easily  occur  and  has  frequently  occurred 
in  the  use  of  compressed  air.  The  earlier  experimenters  in 
this  line  all  encountered  it,  and  most  of  them  on  account 
of  it  at  once  dropped  compressed  air  as  a  practicable  power- 
transmitter,  and  the  freezing  up  of  compressed  air  has 
remained  a  formidable  bugaboo  among  otherwise  intelligent 
mechanics  to  this  day. 

The  best  way  to  do  in  a  case  like  this  is  first  of  all  to 
have  a  good  look  at  it  all  around  in  broad  daylight.  It 


14^  COMPRESSED   AIR. 

would  seem  to  be  worth  while  to  get  together  where  we  can 
see  them  the  principal  facts  of  the  case,  so  that  we  may  be 
able  to  understand  the  conditions  under  which  the  freezing 
up  occurs,  whether  it  must  always  accompany  the  use  of 
compressed  air,  and,  if  not,  the  combination  of  conditions 
under  which  all  danger  of  freezing  up  may  be  successfully 
avoided. 

Intelligent  mechanics  to  maintain  their  up-to-date  in- 
telligence must  be  wide-awake  and  fully  informed,  and  such 
should  know  that  in  these  days  compressed  air  is  widely 
used  not  only  without  freezing  up,  but  also  without  any 
reheating  or  other  special  device  for  preventing  it.  Com- 
pressed-air locomotives,  probably  hundreds  of  them,  are 
constantly  used,  in  mines  and  elsewhere,  without  any  re- 
heating of  the  air  and  without  freezing  up,  and  the  builders 
of  those  locomotives  will  absolutely  guarantee  them  to  do  it 
every  time.  Rock  drills  by  the  thousand  and  pumps  and 
hoisting-engines  without  number  are  run  by  compressed  air 
without  reheating  it  and  without  freezing  up. 

It  must  be  evident  that  for  freezing  up  to  occur  two 
things  are  essential,  and  neither  alone  could  have  any  effect 
toward  producing  such  a  result.  The  free  moisture  must 
be  present  and  accumulating,  and  the  temperature  of  the 
air  where  the  freezing  up  is  to  occur  must  be  below  the 
freezing-point.  The  moisture  alone  can  cause  no  trouble 
as  long  as  the  temperature  continues  high  enough.  It 
will  simply  be  carried  along  with  the  air  and  will  be  dis- 
charged with  it.  So,  too,  a  low  temperature  of  the  air  in 
the  passages  at  any  time  will  not  freeze  up  anything  as 
long  as  there  is  no  free  moisture  present  to  be  frozen. 

We  may  say  generally  that  air  always  contains  moisture. 
Its  capacity  for  moisture  is  determined  by  the  combined 


THE  FREEZING    UP   OF  COMPRESSED    AIR.      149 

conditions  of  pressure  and  temperature  to  which  it  is  at  the 
time  subjected.  Changes  either  of  pressure  or  of  tempera- 
ture immediately  change  the  capacity  of  the  air  for  water, 
and,  supposing  the  air  to  be  saturated  with  water,  whenever, 
either  through  increase  of  pressure  or  through  decrease  of 
temperature,  the  capacity  of  the  air  for  water  is  reduced, 
the  excess  of  water  is  dropped.  At  constant  temperature 
the  capacity  of  air  for  water  seems  to  be  inversely  as  its 
absolute  pressure.  By  another  mode  of  stating  this  it  may 
be  said  that  the  capacity  of  the  air  for  water  is  independent 
of  its  pressure  or  density.  It  is  so  stated  by  parties  who 
have  an  eminent  right  to  speak  upon  the  subject ;  and  the 
statement  is  correct  if  rightly  understood,  but  it  is  apt  to 
be  misleading.  At  uniform  temperature  a  given  volume  of 
air  implies  a  capacity  for  a  certain  weight  of  water,  whether 
the  air  be  at  a  pressure  of  i  atmosphere  or  of  100  atmos- 
pheres; but  if  the  air  has  been  compressed  from  a  press- 
ure of  i  atmosphere  to  a  pressure  of  100  atmospheres,  or 
if  its  volume  has  been  reduced  from,  say,  100  cubic  feet 
to  i ,  cubic  foot,  or  in  that  proportion,  its  capacity  for 
water  has  really  been  reduced  to  one  hundredth  of  its 
original  capacity;  and  if  the  air  before  the  compression 
was  saturated  with  water,  then  after  the  compression,  and 
after  it  has  fallen  to  its  original  temperature,  it  must  have 
dropped  somewhere  during  the  operation  -j^  of  the  water 
that  it  originally  carried. 

At  whatever  pressure  the  air  may  be  changes  of  temper- 
ature immediately  affect  the  capacity  of  the  air  for  carrying 
water.  The  water-carrying  capacity  of  the  air  seems  to  be 
as  sensitive  to  temperature  as  to  pressure.  We  know  very 
distinctly  the  general  fact  that  the  hotter  the  air  the  greater 
its  capacity  for  moisture  ;  but  there  seem  to  be  little  satis- 


ISO  COMPRESSED   AIR. 

factory  data  as  to  the  quantity  of  water  that  will  be  carried 
by  compressed  air  under  different  conditions  of  tempera- 
ture. The  absence  of  such  data,  however,  need  not 
seriously  cripple  us  in  our  quest. 

In  the  operation  of  air-compression  the  heating  of  the 
air,  and  the  increase  of  water  capacity  thereby,  seems  to 
keep  pace  with  and  to  compensate  for  the  reduction  of 
water  capacity  consequent  upon  the  reduction  of  volume, 
and  we  never  hear  of  any  trouble  from  liberated  water  in 
the  compressing  cylinder,  but  after  the  air  leaves  the  com- 
pressor the  water  begins  to  make  itself  known,  and  all  the 
world  hears  of  it.  As  the  air  leaves  the  compressor  it  is 
usually  quite  hot,  and  even  at  the  high  temperature  the  air  is 
usually  saturated,  or  nearly  saturated,  with  water.  As  the  air 
cools  the  water  begins  at  once  to  be  released,  and  before  it 
is  thoroughly  cooled  considerable  water  is  generally  depos- 
ited. Changes  in  the  meteorological  conditions,  or  in  the 
original  humidity  of  the  air  as  it  enters  the  compressing 
cylinder,  of  course  change  the  amount  of  water  precipitated 
by  the  air  after  compression,  and  all  who  have  experience 
with  compressed  air  find  that  on  this  account  the  air  carries 
and  deposits  more  water  at  some  times  than  at  others. 

Many  amateurs  and  experimenters  have  -encountered 
trouble  from  the  freezing  up  of  the  air  on  account  of  taking 
the  air  immediately  from  the  compressor,  before  it  has  been 
completely  cooled,  or,  if  cooled,  by  neglecting  to  drain  off  all 
the  liberated  water  from  the  pipes  before  using  the  air.  The 
experience  thus  obtained  embodied  an  important  lesson  if 
it  could  have  been  learned,  but  the  lesson  has  been  too 
often  misread,  and  the  interpretation  of  the  freezing  up 
phenomenon  has  been  an  incorrect  one.  When  an  air- 
motor  or  an  engine  driven  by  compressed  air  "  freezes  up," 


X" 

THE  FREEZING    UP   OF  COMPRESSED   AIR.      \t>\ 

usually  by  the  choking  of  the  exhaust  passages,  the  general 
impression  among  mechanics  is  that  the  water  is  precipi- 
tated by  the  air  at  the  moment  when  the  freezing  occurs  ; 
but  the  fact  usually  is  that  the  water  is  deposited  in  the 
pipes  by  the  air  before  the  motor  or  engine  is  reached, 
and  the  water  is  then  carried  along  as  entrained  water  by 
the  friction  of  the  air,  and  when  the  temperature  of  the  air 
falls  below  the  freezing-point,  on  account  of  its  expansion  in 
the  cylinder  or  at  the  exhaust,  the  water,  being  present  and 
in  contact  with  the  cold  air,  is  of  necessity  frozen. 

The  general  practice  of  the  day  in  the  compression  and 
transmission  of  air  does  not  seem  to  make  adequate  provi- 
sion for  disposing  of  the  water  deposited  by  the  air  while 
cooling.  As  the  air  leaves  the  compressor  it  is  usually 
quite  hot,  and  even  at  the  high  temperature  it  is  saturated 
or  nearly  saturated  with  water.  As  the  air  cools  it  begins 
at  once  to  lose  its  capacity  for  water,  and  some  of  the  water 
is  dropped  and  continues  to  be  deposited  as  long  as  the  air 
continues  to  cool.  In  connection  with  the  compressor,  and 
usually  quite  near  it,  a  receiver  or  reservoir  of  considerable 
capacity  is  provided,  the  most  important  function  of  which 
is,  or  is  assumed  to  be,  that  of  collecting  the  water  that  may 
be  precipitated  by  the  compressed  air.  In  too  many  cases 
this  receiver  fails  of  its  mission,  or  only  partially  collects  the 
water  from  the  air,  because,  if  the  compressor  is  working 
constantly  and  rapidly,  as  it  usually  does,  the  air  goes 
through  the  receiver  and  out  of  it  and  into  the  pipe-line 
before  it  has  time  to  cool.  The  air  after  compression  will 
not  drop  all  of  its  water  until  it  is  thoroughly  cooled,  and 
the  cooler  it  gets  the  greater  will  be  the  quantity  of  water 
liberated  ;  and  when  the  air,  still  under  full  pressure,  has 
reached  the  lowest  temperature  attainable,  means  should  then 


1 52  COMPRESSED   AIR. 

be  provided  for  collecting  the  liberated  water,  or  it  must,  of 
course,  be  carried  along  in  the  pipes  to  make  trouble 
by  freezing  up  where  the  air  is  used,  and  where  the  air 
expands  and  cools  while  doing  its  work  or  upon  its  release. 
With  a  receiver  near  the  compressor,  and  with  hot  air  passing 
through  it,  and  a  pipe-line  long  enough  to  completely  cool 
the  air  before  it  is  used  in  rock  drill,  air-motor,  pump,  or  othei 
constantly  running  machine,  and  with  no  provision  for  dis- 
posing of  the  water  in  the  pipe,  we  should  expect  to  hear  of 
the  machines  freezing  up.  Cases  are  quite  common  where 
a  second  receiver  placed  at  the  farther  end  of  a  pipe-line  has 
effectually  cured  the  freezing  up  by  removing  the  congeal- 
able  liquid. 

A  few  years  ago  many  air-compressors  for  driving  rock 
drills  were  in  use  in  the  United  States — a  large  number  of 
them  upon  the  Croton  aqueduct — which  cooled  the  air  dur- 
ing compression  by  the  injection  of  jets  of  water  into  the  air 
in  the  compressing  cylinder.  Compressors  of  this  style  are 
not  now  built  by  any  firm  in  the  United  States.  The  cylinders 
were  found  to  wear  out  quite  rapidly,  the  compressors  could 
not  be  run  as  fast  as  the  dry  compressors,  and  for  other 
similar  reasons  they  did  not  pay.  They  did,  however,  deliver 
the  compressed  air  decidedly  cooler  than  the  compressors 
now  in  use  deliver  it,  and  it  is  not  surprising  that  it  should 
be  claimed  by  rock-drill  runners,  and  probably  correctly,  that 
those  old  injection  compressors,  with  the  water  intimately 
mingling  with  the  air  during  the  compression,  still  furnished 
drier  air,  and  consequently  air  less  liable  to  "  freeze  up," 
than  the  more  modern  dry  compressors  furnish. 

In  the  process  of  wood-vulcanizing,  for  preserving  wood 
by  cooking  the  sap  in  the  wood,  the  material  to  be  treated 
is  enclosed  in  tight  cylinders  and  subjected  to  an  air  press- 


THE  FREEZING    UP    OF  COMPRESSED    AIR.      1 53 

lire  of  150  pounds  The  air  is  specially  heated  and  made 
to  circulate  around  and  among  the  wood  and  absorb  the 
moisture  that  may  be  liberated  from  it,  so  that  it  is  essential 
to  the  process  that  the  air  should  be  as  dry  as  possible,  and 
the  paradox  occurs  that  to  secure  dry  air  the  wet  or  injec- 
tion type  of  compressor  is  employed. 

I  know  a  certain  iron  mine  which  has  two  air-compressors 
side  by  side,  each  connected  to  deliver  the  compressed  air 
through  the  same  receiver  and  the  same  pipe  to  the  rock 
drills  in  the  mine.  One  of  the  compressors  delivers  its  air 
at  a  temperature  considerably  below  that  of  the  air  from 
the  other  compressor,  say  from  50°  to  100°  lower,  neither 
of  the  compressors  being  of  the  injection  type,  and  it  is  con- 
stantly noted  that  the  men  in  the  mine  operating  the  drills 
can  immediately  tell  which  compressor  is  running  by  the 
relative  humidity  of  the  air  supplied.  The  compressor  which 
delivers  the  coolest  air  of  course  delivers  the  driest  air. 

When  the  air  is  completely  saturated  with  water,  contact 
with  water  will  not  make  it  any  wetter.  The  water  in  the 
injection  compressor  did  not  wet  the  air,  for  it  was  as  wet 
as  it  could  be  ;  and  as  that  water  enabled  the  compressor  to 
deliver  the  air  at  a  lower  temperature  than  the  dry  com- 
pressor would  deliver  it,  the  air,  simply  because  it  was 
cooler,  actually  emerged  from  the  compressor  bearing  less 
moisture  than  the  air  emerging  at  the  same  pressure,  but 
at  a  higher  temperature,  from  the  dry  compressor.  If  no 
means  were  provided  for  draining  the  surplus  water  from 
the  air,  except,  in  either  case,  the  receiver  located  near  the 
compressor,  the  cooler  air  passing  the  receiver  would  carry 
the  less  amount  of  water  into  the  pipe  and  through  it ;  but 
if,  in  each  case,  after  the  air  had  traversed  the  pipe  a  suffi- 
cient distance  to  have  become  thoroughly  cooled  another 
receiver  or  drainage  chamber  had  been  provided,  there  is 


I $4  COMPRESSED   AIR. 

no  reason  why,  after  emerging  from  the  chamber,  the  air  in 
each  case  being  at  the  same  pressure  and  temperature,  the 
one  should  carry  any  more  water  than  the  other.  To  get 
rid  ot  all  trouble  from  water  in  the  air,  and  the  possible 
freezing  of  it,  care  should  be  taken  that  when  the  air  passes 
a  point  where  it  is  still  at  full  pressure  and  has  reached 
its  lowest  temperature,  such  means  of  drainage  shall  be 
provided  that  none  of  the  liberated  water  shall  be  carried 
into  and  along  the  pipes  beyond  that  point. 

The  possible  freezing  up  that  we  have  been  contemplat- 
ing thus  far  along  in  this  chapter  is  where  water  is  present 
by  deposition  from  the  compressed  air,  and  where  a  low 
temperature  is  caused  by  the  expansion  of  the  air,  and  the 
freezing  of  the  water  ensues  by  contact.  Another  mode  of 
freezing  up  is  experienced  where  the  freezing  is  accom- 
plished not  by  the  air  that  has  been  compressed,  but  by  the 
external  atmosphere.  In  the  winter  if  compressed  air  at 
low  temperature,  but  still  above  the  freezing-point,  saturated 
with  water,  as  it  is  pretty  sure  to  be,  and  with  the  pipe 
thoroughly  drained  to  a  certain  point,  has  then  to  pass  for 
some  distance  through  a  pipe  exposed  to  a  freezing  atmos- 
phere, it  cannot  fail  to  deposit  some  water,  and  the  freez- 
ing of  the  water  so  deposited  may  soon  choke  -the  pipe.  I 
have  encountered  cases  of  this  kind  more  than  once,  notably 
in  one  of  the  largest  chemical  works  of  the  country,  where 
the  pressure  of  the  air  is  employed  in  lifting  and  transferring 
acids  so  that  they  may  not  be  exposed  to  metallic  contact. 
The  air-pipe  was  carried  through  the  extensive  works  and 
from  building  to  building,  and  in  some  places  between  the 
buildings  where  exposed  to  the  extreme  cold  of  a  sharp 
winter  it  was  choked  up  by  the  accumulation  and  successive 
freezing.  The  only  apparent  lesson  in  this  case  is  to  protect 
the  pipe  from  frost.  A  pipe  conveying  compressed  air  and 


THE  FREEZING    UP   OF  COMPRESSED   AIR.      155 

exposed  to  a  freezing  atmosphere  is  quite  sure  to  choke  up. 
The  deposition  of  the  water  may  proceed  slowly,  but  if  the 
low  external  temperature  continues,  the  accumulation  will 
eventually  reduce  the  air-channel,  or  even  close  it  entirely. 
This  result  is,  of  course,  chargeable  to  the  weather,  and  not 
to  the  innate  frigorific  malignity  of  the  compressed  air. 

The  pressure  at  which  the  compressed  air  is  transmitted, 
ind  eventually  used,  has  an  important  bearing  upon  the 
question  of  its  freezing  up  in  use.  If  the  air  is  transmitted 
only  short  distances,  and  at  comparatively  low  pressures 
the  probabilities  of  freezing  up  are  much  greater  than  if 
high  pressures  are  employed  and  if  the  distance  of  trans- 
mission is  at  least  sufficient  to  allow  a  thorough  cooling  and 
drainage  of  the  air  while  under  the  full  pressure.  In  the 
use  of  low-pressure  air  for  any  service,  of  course  a  larger 
volume  of  free  air  is  used  to  furnish  any  given  power,  and 
the  larger  volume  of  air  implies  the  presence  of  a  greater 
quantity  of  water  in  suspension,  and  the  lower  pressure  em- 
ployed affords  less  opportunity,  or  no  opportunity,  for  ex- 
tracting the  water,  and,  as  a  fact  of  experience,  most  of  the 
freezing-up  trouble  that  is  encountered  is  from  air  that  is 
used  at  comparatively  low  pressure. 

There  are  many  considerations,  which  I  need  not  enu- 
merate here,  to  commend  the  use  of  air  at  high  pressures, 
and  not  the  least  among  those  considerations  is  the  practical 
immunity  from  freezing  up  that  is  thereby  secured.  This 
may  be  readily  understood,  Say  that  air  is  compressed  to 
1000  pounds  gauge,  or,  say,  70  atmospheres,  either  that 
smaller  pipes  may  be  used  for  long-distance  transmission, 
or  that  smaller  receivers  may  be  used  for  the  storage  of  air 
upon  a  street  railway  motor,  and  say  that  the  air  is  admitted 
to  the  motor  at  100  pounds  pressure.  If  while  the  air  is  at 
1000  pounds  it  is  thoroughly  cooled  and  drained,  it  is  evi- 


156  COMPRESSED    AIR. 

dent  that  when  that  air  is  expanded  to  100  pounds,  and 
has  been  allowed  to  regain  its  normal  temperature,  if  the 
air  was  just  saturated  with  moisture  when  at  1000  pounds 
pressure  and  normal  temperature,  when  it  has  expanded  to 
more  than  eight  times  its  former  volume  it  can  be  only  one 
eighth  saturated,  and  no  water  can  be  deposited  by  it  in 
expanding  from  100  pounds  downward,  and  however  low 
the  temperature  may  fall  there  can  be  no  freezing  up. 

A  valuable  use  of  compressed  air  is  for  the  transmission 
of  packages  or  mail  matter  through  suitable  tubes  from  one 
station  to  another.  In  this  pneumatic  transmission  service 
some  trouble  has  been  experienced  in  the  winter  from  the 
accumulation  of  ice  in  the  pipes.  As  the  pressure  employed 
is  low  the  freezing  up  might  be  prevented  by  compressing 
all  the  air  to  a  pressure  considerably  higher  than  required 
and  cooling  and  draining  it  while  under  that  higher  pres- 
sure. Then  after  passing  a  reducing  valve  to  the  low  pres- 
sure for  use  the  air  would  be  dry  and  could  not  deposit 
moisture  to  be  frozen. 

Compressed  air  is  often  used  in  caissions  for  bridge  piers 
and  kindred  uses  where  the  compressor  is  so  near  the  cais- 
sion  that  the  air  in  transmission  does  not  become  as  cool  as 
it  should  be,  and  the  men  find  the  warm  atmosphere  very 
oppressive  and  are  unable  to  do  as  much  work  as  should  be 
expected.  The  service  pipes  are  sometimes  cooled  by  pass- 
ing them  through  water,  but  even  then  the  air  in  the  caissons 
is  warmer  than  it  should  be  for  vigorous  work.  In  this  case 
also  if  the  air  were  compressed  to  a  pressure,  say,  20  or  30 
pounds  higher  than  required,  then  cooled  as  well  as  possible 
by  passing  the  pipes  through  cold  water,  and  after  that  ad- 
mitted to  the  caisson  through  a  pressure -reducer  adjusted 
to  the  desired  pressure,  it  would  then  be  cool  enough,  or  it 
might  even  be  made  cooler  than  required. 


CHAPTER  XVIL 
REHEATING  COMPRESSED  AIR 

WHILE  air  at  low  temperature  has  a  comparatively  small 
cooling  effect  upon  water  or  upon  whatever  may  be  in  con- 
tact with  it,  the  fact  inversely  applied  is  of  advantage  in  the 
use  of  compressed  air  for  power-transmission.  Jt  requires 
comparatively  little  heat  to  raise  the  temperature  of  air 
rapidly.  It  is  well  known  that  after  the  transmission  ol 
compressed  air  to  the  point  where  it  is  to  be  employed  a 
considerable  saving  in  the  cost  of  the  available  power  is 
effected,  theoretically  p.t  least,  by  reheating  the  air  before  it 
is  used.  While  many  have  called  attention  to  this  matter 
in  various  ways,  few  have  given  us  any  definite  and  reliable 
data  regarding  it.  Little  is  generally  known  as  to  the  actual 
economy  of  such  a  practice,  or  of  the  conditions  under 
which  it  is  practicable 

It  may  easily  be  shown  that  where  a  certain  volume  of 
air  has  been  compressed  to  a  given  pressure,  and  has  by 
transmission  or  storage  resumed  approximately  its  normal 
temperature,  if  that  air  is  then  reheated  and  thereby  ex- 
panded, the  additional  volume  of  compressed  air  resulting 
from  the  expansion  is  produced  by  an  expenditure  of  heat 
much  lower  than  the  original  volume  of  compressed  air  was 
produced  for,  and  by  a  much  lower  expenditure  of  heat 
than  is  required  to  produce  an  equal  volume  of  steam  The 
actual  figures  in  the  case,  all  theoretical,  are  as  follows 

157 


1 58  COMPRESSED   AIR. 

Weight  of  i  cu.  ft.  of  steam  at  75  Ibs.  gauge  =  .2089  Ib. 

Total  units  of  heat  in  i  Ib.  of  steam  at  75  Ibs.  from 
water  at  60°  =  1151 

Total  units  of  heat  in  i  cu.  ft.  of  steam  at  75  Ibs.  =  1151 
X  .2089  =  240.44 

To  produce  by  compression  through  a  steam-actuated 
air-compressor  i  cu  ft  of  compressed  air  at  75  Ibs.  and  6oc 
about  2  cu  ft  of  steam  ot  the  same  pressure  are  required, 
or  the  heat-units  employed  in  producing  i  cu  ft  of  com- 
pressed air  will  be  about  240.44  X  2  —  480  88  heat  units  as 
the  thermal  cost  of  i  cu.  ft.  of  compressed  air  at  the  above 
temperature  and  pressure  The  temperature  and  volume 
of  the  air  as  it  leaves  the  compressor  will  be  considerably 
higher  than  the  figures  here  assumed,  but  as  the  air  is  in- 
variably stored  tor  a  time,  or  is  transmitted  through  pipes 
to  a  distance,  between  its  compression  and  its  ultimate 
employment,  it  may  be  said  to  always  return  to  its  normal 
temperature  before  it  is  used,  so  that,  whatever  we  may 
have  at  the  compressor,  the  air  as  it  is  delivered  to  the 
motor,  or  whatever  apparatus  may  be  operated  by  it,  will 
have  cost,  as  above  stated.  480.88  heat- units  for  i  cu.  ft  at 
75  Ibs  The  difference  in  the  thermal  cost  of  any  volume 
of  compressed  air  thus  produced  by  mechanical  compres- 
sion and  the  cost  of  any  additional  volume  of  air  that  may 
result  from  the  subsequent  reheating  of  the  air  is  very 
striking. 

The  weight  of  i  cu.  ft.  of  free  air  at  60°  =  .076  Ib. 

Weight  of  i  cu.  ft.  of  compressed  air  at  75  Ibs.  and 
60°  =  .456 

Units  of  heat  required  to  double  the  volume  of  i  Ib.  of 
air  at  60°  =  123.84 

Units  of  heat  required  to  double  the  volume  of  i  cu.  ft. 


REHEATING    COMPRESSED   AIR.  159 

of  compressed  air  at  75  Ibs.  and  60°  =  123.84  X  ,456  = 

56-47- 

Cost  of  i  cu.  ft.  of  superheated  compressed  air  3175  ^s- 
compared  with  the  cost  of  i  cu.  ft.  of  compressed  air  as 
produced  by  ordinary  compression: 

480.88  .-56.47  :  :  i  :  .1174. 

Here  we  see  that  the  cost  in  heat-units  of  the  volume  ot 
air  produced  by  the  reheating  is  less  than  one  eighth  of  the 
cost  of  the  same  volume  produced  by  compression.  Upon 
this  showing  the  matter  is  certainly  worth  looking  into, 
because  if  there  is  such  a  possible  opening  for  the  econom- 
ical application  of  heat  to  the  development  of  power,  we 
ought  to  know  it  and  avail  ourselves  of  it. 

The  operation  of  reheating  compressed  air  is  correctly  so 
termed.  It  is,  in  fact,  a  case  of  doing  work  over  again,  or 
of  replacing  in  the  air  heat  that  has  been  lost  by  it  in  previ- 
ous operations.  It  must  be  confessed  that  the  presumption 
is  all  against  our  finding  much  profit  in  this  direction 
There  are  not  many  places  in  life  where  it  pays  to  do  our 
work  a  second  time  There  is,  as  we  have  seen,  practically 
no  air-compression  without  heating  the  air  by  the  operation, 
and  there  is  no  transmission  of  air  after  compression  with- 
out its  cooling  to  very  near  its  original  temperature  If  the 
air  could  go  immediately  from  the  compressing  cylinder  into 
the  motor  cylinder,  where  it  does  its  work,  without  losing 
any  of  its  heat,  it  would  have  the  same  effective  power  as 
it  would  have  after  long-distance  transmission  and  cooling 
and  reheating,  and  without  the  additional  cost  of  that  re- 
heating. While  we  are  saying  in  all  good  faith  that  there 
is  little  loss  of  power  in  the  transmission  of  compressed  air 


160  COMPRESSED    AIR. 

to  considerable  distances,  and  that  the  difference  in  the 
pressure  of  the  air  at  the  two  ends  of  a  long  pipe  necessary 
to  overcome  the  friction  and  maintain  the  flow  is  but  small, 
and  that  it  is,  to  a  great  extent,  compensated  for  by  the 
increased  volume  at  delivery,  the  fact  still  is  that  there  is  a 
great  loss  of  power  in  the  transmission  of  the  air,  if  we 
reckon  from  the  moment  when  compression  ceases,  on  ac- 
count of  the  inevitable  cooling  of  the  air.  Still  this  loss  is 
not  properly  chargeable  to  the  transmission,  for  no  matter 
how  far  the  air  may  be  transmitted  the  cooling  is  all  accom- 
plished before  the  air  has  travelled  very  far  if  the  pipes  are 
of  proper  size.  Supposing  air  to  be  transmitted  ten  miles, 
it  must  be  conveyed  with  considerable  rapidity  if  it  does 
not  get  down  to  normal  temperature  before  the  end  of  the 
first  quarter  of  a  mile.  • 

As  the  volume  of  air  under  any  constant  pressure  varies 
directly  as  the  absolute  temperature,  it  follows  that  to 
double  the  volume  by  heating  the  air  its  absolute  tempera- 
ture must  be  doubled.  The  air  being  at  60°,  its  absolute 
temperature  will  be  60  +  461  =  521,  and  double  this  will 
be  521  X  2  =  1042,  the  absolute  temperature  required. 
This  by  the  thermometer  will  be  1042  —  461  =  581°.  As 
this  is  the  temperature  that  is  required  for  the  air  when 
delivered  into  the  motor,  and  actually  beginning  its  work, 
it  will  be  necessary,  on  account  of  the  ease  and  rapidity 
with  which  it  cools,  to  heat  the  air  considerably  higher  than 
this  theoretical  temperature.  It  is  one  thing,  and  an  easy 
one,  to  heat  the  air,  while  it  is  a  very  different  and  a  very 
difficult  thing  to  keep  it  hot.  To  avoid  all  loss  of  heat  it 
would  be  necessary,  not  only  to  keep  the  pipe  which  con- 
veyed the  air  constantly  hot,  but  also  the  cylinder  in  which 


REHEATING    COMPRESSED   AIR.  l6l 

it  was  used,  or  it  would  be  cooled  before  it  began  to  do  its 
work.  In  one  case  within  my  experience,  where  com- 
pressed air  was  reheated,  and  its  absolute  temperature  was 
increased  at  the  heater  38  per  cent,  and  where,  of  course, 
its  theoretical  increase  of  volume  was  the  same,  the  actual 
increase  of  power  realized  was  only  12  per  cent.  In  this 
case  the  air  was  transmitted  after  the  reheating  about  20 
feet,  the  pipe  was  not  covered,  and  no  precautions  were 
taken  to  prevent  loss  of  heat  by  radiation.  The  volume 
of  air  transmitted  was  sufficient  to  develop  between  20 
and  30  horse-power.  The  theoretical  temperature  re- 
quired to  double  the  volume  of  compressed  air  at  60°  being 
581°,  the  actual  temperature  required  at  the  heater  under 
the  most  favorable  .conditions  in  order  to  have  a  double 
volume  of  air  available  in  the  motor  will  not  be  less  than 
800°,  and  this  is  a  temperature  that  it  is  practically  impos- 
sible to  employ  and  maintain,  and  we  may  as  well  give  up 
all  thought  of  doubling  the  volume  of  compressed  air  by 
reheating  it  and  of  realizing  the  promised  economy  of  the 
operation. 

If  instead  of  doubling  the  volume  we  only  attempt  to 
increase  it  by  one  half,  or  50  per  cent,  which  it  is  practica- 
ble to  do,  the  required  theoretical  temperature  (absolute) 
will  be  521  +  50  per  cent  =  782,  and  782  —  461  =  321°, 
the  sensible  temperature  required.  Adding  enough  to  this 
to  allow  for  the  intermediate  cooling,  the  actual  temperature 
required  should  probably  be  not  less  than  450°.  The  tem- 
perature of  the  air  before  the  reheating  being  assumed  to  be 
60°,  the  increase  of  temperature  will  be  450°  —  60°  =  390°. 
As  we  saw  above  that  it  required  56.47  heat-units  to  raise 
the  tempeature  of  i  cu.  ft.  of  compressed  air  at  75  Ibs. 
gauge  pressure  from  60°  to  581°,  the  actual  increase  of  tem- 


1 62  COMPRESSED   AIR. 

perature  being  581  —  60  =  521,  it  follows  that  to  raise  the 
temperature  390°  will  require  : 

521  1390  ::  56.47  142.27. 

Then  if  the  first  cubic  foot  of  compressed  air  costs  480.88 
heat-units  for  its  compression,  and  if  the  additional  half  of 
a  cubic  foot  produced  by  reheating  costs  42.27  heat-units, 
the  total  cost  of  if  cu.  ft.  under  the  reheating  system  will 
be  480.88  +  42.27  =  523.15,  and  the  cost  per  cubic  foot  at 
this  rate  will  be  523.15  -=-  i|  =  348.76  heat-units.  The 
relative  cost  in  heat-units  of  i  cu.  ft.  of  compressed  air  pro- 
duced by  compression  alone,  and  of  a  cubic  foot  resulting 
from  compression  and  reheating,  will  be  : 

480.88  :  348.76  : :  i  :  .72. 

From  this  it  appears  that  the  gain  by  reheating  com- 
pressed air  sufficiently  to  increase  its  effective  volume  50 
per  cent  will  be  28  per  cent.  The  more  fair  and  correct 
way  to  state  this,  however,  will  be  to  reverse  it : 

.72  :  i  : :  i :  1.38. 

We  may  say,  then,  that  the  total  fuel  applied  with  the 
reheating  system  will  yield  38  per  cent  higher  results  than 
are  to  be  realized  without  the  reheating.  This  seems  to  be 
very  near  the  maximum  that  can  be  attained  in  the  way  of 
economy  by  reheating  dry  compressed  air. 

It  is  not  always,  nor,  indeed,  often,  that  the  reheating  of 
compressed  air  is  practicable  or  possible.  In  a  valuable 
report  upon  compressed-air  appliances  by  a  committee  of 
the  Master  Car-Builders'  Association,  1894,  they  say:  "It 
was  reported  by  the  manufacturers  of  air-appliances  that 
superheated  compressed  air  used  in  air-lifts,  jacks,  engines, 


REHEATING    COMPRESSED   AIR.  163 

etc.,  increases  the  efficiency  fully  50  per  cent,  but  your 
committee  was  unable  to  make  tests  or  to  procure  reliable 
data,  etc."  The  "manufacturers  of  air-appliances,"  quoted 
by  the  committee,  either  were  not  responsible  for  their 
words  or  they  did  not  know  what  they  were  talking  about. 
Bearing  in  mind  the  facility  and  rapidity  with  which  heated 
air  in  transmission  loses  its  heat,  it  is  idle  to  think  of  ever 
heating  compressed  air  except  for  continuously  running 
motors,  and  then  by  heaters  very  close  to  the  motcrs.  In 
Paris,  where  25,000  horse-power  is  employed  for  general 
compressed-air  service,  the  air  in  some  instances  is  used  to 
run  engines  that  were  formerly  run  by  steam,  the  original 
boiler  that  supplied  the  steam  for  the  engine  being  retained 
as  a  heater  and  reservoir  for  the  air.  That  is  all  right,  and 
wherever  any  motor  or  engine  is  to  be  run  without  inter- 
ruption a  heater  for  the  air  should  certainly  be  employed ; 
but  at  the  end  of  this  volume  is  a  list  of  two  hundred  dif- 
ferent and  distinct  uses  of  compressed  air  in  not  one  of 
which  would  it  be  practicable  or  anything  but  a  losing  op- 
eration to  try  to  heat  the  air.  In  the  United  States  at  the 
present  time  there  is  probably  not  one  case  in  a  thousand 
where  compressed  air  is  employed  and  where  one  cent  of 
profit  could  be  realized  from  reheating  the  air.  It  is  to  be 
regretted  that  American  compressed-air  practice  is  not  so 
far  developed,  or  developed  upon  such  lines,  as  to  make  the 
economy  of  reheating  the  air  before  its  use  more  readily 
and  generally  available.  When,  by  and  by,  compressed  air 
comes  to  be  used  for  what  we  may  term  legitimate  power- 
transmission,  and  is  employed  to  drive  small  motors  and 
motors  not  so  small  with  the  established  functions  of  the 
steam-engine,  then  the  reheater  will  find  its  field  of  useful- 
ness. 


164 


COMPRESSED   AIR. 


In  connection  with  this  topic  it  is  hoped  that  the  accom- 
panying diagrams,  Figs.  24  and  25,  will  be  of  some  interest 
and  value.  Fig.  24  shows  the  increase  of  volume  accom- 


panying  the  heating  or  reheating  of  compressed  air.  The 
air  is  assumed  to  be  heated  from  the  several  initial  temper- 
atures of  o°,  32°,  60°,  and  100°,  the  pressure  remaining 
constant  during  the  operation  represented.  The  relative 


REHEATING    COMPRESSED    AIR. 


I65 


volume  at  any  temperature  is  indicated  by  the  height  of 
the  vertical  line  corresponding  with  that  temperature,  the 
height  from  AB  to  CD  representing  one  volume,  and  each 

Gage  Pressures 


o       S       8       K 


iS-Ii 


Absolute  Fresauret 

horizontal  line  above  that  indicating,  successively,  an 
additional  one  tenth  of  volume.  When  the  line  EF  is 
reached,  the  original  volume  is  doubled.  Figures  below 
the  base-line  AB  indicate  the  sensible  temperatures 


1 66  COMPRESSED   AIR. 

Fahrenheit,  and  the  figures  above  the  upper  line  indicate 
the  corresponding  absolute  temperatures. 

Fig.  25  shows  the  increase  of  pressure  only  caused  by 
the  heating  of  compressed  air,  the  volume  being  constant. 
The  air  is  assumed  to  be  heated,  as  in  Fig.  24,  from  the 
several  initial  temperatures  of  o°,  32°,  60°,  and  100°,  and 
also  from  a  number  of  different  initial  pressures.  The 
pressures  are  indicated  by  the  several  horizontal  lines,  the 
vertical  distance  between  any  two  adjacent  lines  represent- 
ing approximately  i  atmosphere.  The  figures  at  the  left  of 
the  diagram  indicate  the  gauge  pressures,  and  the  figures  at 
the  right  the  absolute  pressures.  The  temperatures  are  in- 
dicated as  in  the  previous  diagram. 


CHAPTER  XVIII. 
COMPRESSED  AIR  FOR  PUMPING. 

THE  air-compressor  owes  its  modern  development  to  the 
demands  of  the  rock  drill,  in  its  various  forms  and  appli- 
cations, more  than  to  any  other  single  cause.  The  larg- 
est builders  of  air-compressors  for  general  use  first  engaged 
in  their  manufacture  to  supply  the  air  to  the  rock  drills 
that  they  were  building.  All  of  the  rock  drills  in  all  of 
the  mines,  and  in  every  tunnel  that  is  being  driven,  and  in 
every  shaft  that  is  being  sunk,  we  may  say,  are  and  appar- 
ently must  be  driven  by  compressed  air.  Nobody  seems  to 
inquire,  and  nobody  knows  very  clearly,  whether  or  not  the 
power  employed  in  operating  rock  drills  is  applied  economi- 
cally or  not.  The  conditions  under  which  the  drills  are 
employed  and  the  nature  of  their  work  are  all  inimical  to 
economy.  It  is  impossible  to  measure  the  actual  work 
done  by  a  rock  drill.  It  is  enough  that  their  work  pays, 
and  that  there  is  little  annoyance  or  anxiety  involved  in 
keeping  them  going  all  right. 

But,  taking  the  mines  as  they  run,  the  operating  of  the 
drills  is  only  one  of  several  uses  of  power  in  mining  opera- 
tions, and  not  the  largest  of  these,  although  to  some  it  may 
seem  to  be  the  most  important.  Hoisting,  including  haul- 
age, and  pumping,  either  of  them,  upon  the  average,  re- 
quires more  power  than  is  required  for  the  drilling.  Good 
steam-engines  at  the  surface  may  generally  be  employed 

167 


1 68  COMPRESSED    AIR. 

for  the  hoisting,  and  they  do  so  well  that  we  need  not  here 
trouble  ourselves  much  about  them.  With  the  use  of 
power  for  mine-pumping  it  is  all  very  different.  The 
pumps  must  generally  be  located  where  the  water  is,  and, 
if  steam  is  used  to  drive  them,  far  away  from  the  boiler 
plant ;  and  it  so  happens  that  to-day  probably  the  most 
wasteful  use  of  steam  to  be  found  anywhere  is  to  be  found 
where  mine  pumps  are  operated. 

This  certainly  ought  not  to  be  so.  The  operation  of 
pumping  is  one  of  the  most  favorable  ever  found  for  the 
economical  application  of  power.  The  marine-engine  and 
the  water-works  pumping-engine  divide  the  honors  as  exam- 
ples of  the  highest  economy  in  the  wide  range  of  modern 
engineering  practice.  The  stationary  engine,  except  under 
the  most  favorable  conditions,  does  not  equal  their  per- 
formance. The  success  of  the  marine-engine  and  of  the 
pumping-engine  is  to  be  attributed  to  the  one  operating 
condition  that  they  have  in  common,  and  that  is  not  found 
elsewhere.  They  have  constantly  uniform  work  to  do. 
The  pumping  of  water  from  our  mines  should  also  be  eco- 
nomically done,  because  in  this  service,  almost  as  much  as 
with  the  water-works  pump,  the  height  of  the  lift  is  practi- 
cally constant  and  unvarying.  It  is  not  necessary  to  say 
how  different  is  the  actual  result  in  the  case  of  the  mine 
pump.  Waste  of  power  and  expense  for  repairs,  and  for 
duplicate  machinery,  are  the  natural  and  inevitable  accom- 
paniments of  steam-pumping  in  deep  or  extensive  mines. 
The  trouble  of  course  is  principally  in  carrying  the  steam 
so  far. 

Everybody  should  know  that  steam  need  not  be  employed 
under  such  conditions.  Compressed  air  stands  ready  to  do 
this  work  and  to  do  it  more  cheaply  and  more  satisfactorily 


COMPRESSED   AIR  FOR   PUMPING.  169 

than  it  can  be  done  by  any  other  means.  It  is  to  be  con- 
fessed that  air,  where  it  has  been  employed  for  mine-pump- 
ing, has  not  by  any  means  made  as  favorable  an  exhibit  as 
it  should  have  made,  and  has  not  made  the  progress  that  it 
should  toward  universal  adoption  for  this  service.  The 
why  of  it  is  easily  found. 

Here  is  a  practical  example  in  the  case  of  what  is  said  to 
be  the  largest  coal-producing  mine  in  the  United  States. 
Of  course  they  have  a  lot  of  pumping  to  do  there,  and  of 
course  they  use  for  it  the  always  handy  steam-pump.  In 
this  mine  the  steam  is  conveyed  to  the  pumps  by  one  line 
about  a  mile,  and  by  another  line  about  a  mile  and  a  half. 
The  steam  condenses  all  along  its  journey,  and  with  high 
pressure  at  the  boilers  there  is  low  pressure,  and  often  too 
low  pressure,  at  the  pumps.  What  starts  from  the  boilers 
as  steam  is  mostly  water  when  it  gets  to  the  pumps,  and 
they  are  operated  by  combined  hydraulic  and  vaporous  ac- 
tion, the  simple  steam-pump  thus  becoming  what  might  be 
termed  a  diabolically  reversed  compound.  The  great  feat- 
ure of  the  American  steam-pump  is  that  it  will  actually  go. 
That  it  will  go  in  the  mine  makes  it  also  go  in  the  market. 
It  is  to  be  recorded  to  its  credit  (?)  that  it  makes  it  possible 
to  do  what  it  should  be  impossible  to  do.  And  so  we  find 
in  this  mine  eight  of  these  pumps,  in  various  grades  of  in- 
efficiency, and  the  steam  is  carried  a  mile  or  a  mile  and  a 
half  to  make  them  go.  The  steam  leaks  everywhere,  the 
roofs  are  rotting  and  tumbling  in  under  the  combined  ac- 
tion of  the  heat  and  the  moisture,  sometimes  joints  blow 
out  or  pipes  break,  men  are  scalded,  passages  are  blocked, 
ventilation  is  stopped,  gangs  of  repairers  are  constantly  em- 
ployed, but  the  troubles  keep  increasing.  Something  has 


I^O  COMPRESSED    AIR. 

to  be  done  about  it.  It  will  not  do  to  use  steam  any 
longer  here. 

When  it  comes  to  this  point  it  is  very  unfortunate  that 
this  is  a  coal  mine.  A  little  item  in  the  cost  of  a  plant  to 
be  installed  will  outweigh  all  considerations  of  fuel  or  of 
power  economy.  The  decisions  of  coal-mine  managers  are 
therefore  no  suitable  precedents  for  the  miners  who  must 
feed  their  boilers  with  greenbacks.  Electricity  or  com- 
pressed air,  either  of  them,  stands  ready  to  take  up  this  job 
of  pumping,  and  get  rid  of  this  steam  nuisance  in  the  mine. 
But  electricity  has  no  chance  here,  because  new  pumps 
would  have  to  be  bought,  and  there  would  be  also  the  wir- 
ing of  the  mine,  while  compressed  air  is  so  accommodating 
that  it  agrees  to  use  the  old  pumps  and  the  old  pipes,  and 
no  one  need  doubt  that  the  pumps  will  actually  go.  Com- 
pressed air  is  accordingly  adopted,  not  upon  its  merits,  but 
because  it  will  not  cost  so  much  to  put  it  in  ;  and  electric- 
ity is  not  permitted  to  make  an  unseemly  exhibition  of 
itself. 

Few  perhaps  realize  how  peculiar,  and,  indeed,  unique, 
have  been  the  conditions  under  which  electricity  has  been 
spread  abroad.  In  every  case  where  it  has  been  employed 
in  power-transmission  its  installation  has  involved  an  en- 
tirely new  plant  throughout,  each  end  of  the  plant  has  been 
adapted  to  the  other,  and  every  detail  to  the  whole,  so  that 
it  has  never  been  placed  in  a  false  position  or  shown  at  a 
disadvantage,  as  compressed  air  is  almost  invariably  served. 
If  electricity  had  secured  a  chance  at  this  mine,  putting  in 
the  new  electrically-driven  pumps,  as  well  as  the  generators, 
and  with  everything  new,  consistent,  and  complete,  it  would, 
no  doubt,  have  put  on  airs  over  the  results  accomplished, 
as  compared  with  what  compressed  air  has  done  in  some 


COMPRESSED   AIR  FOR  PUMPING.  \T\ 

cases  ;  but  there  would  have  been,  after  all,  really  no  basis 
for  comparison. 

For  supplying  the  compressed  air  instead  of  steam  to  this 
mine,  an  excellent  compressor-plant  is  installed,  a  plant 
that  any  manufacturer  might  be  proud  of  and  might  be 
pardoned  for  bragging  about.  The  performance  of  these 
compressors  is  presumably  as  good  as  that  of  any  compres- 
sors to  be  found  to-day.  But  the  air  furnished  by  the  com- 
pressors is  to  be  used  in  those  abominable  steam-pumps. 
I  have  elsewhere  expressed  my  disgust  at,  and  protested 
against,  the  apparent  indifference  of  the  air-compressor 
builders  as  to  the  uses  to  which  the  air  is  put  after  it  leaves 
the  compressor,  or  as  to  whether  the  air  is  applied  econom- 
ically or  not.  Too  often  the  explanation  in  the  case  is  the 
same  as  in  this  case,  completely  exonerating  the  compres- 
sor builders.  The  conditions  determining  the  adoption  of 
compressed  air  for  this  mine  are  that  the  old  pumps  are  to 
be  used:  Of  course  the  compressor  builders  cannot  afford 
to  kill  their  business  to  save  their  ideals  ;  they  get  a  good 
job,  and  the  compressors  are  put  in  to  drive  those  pumps. 

It  is  not  known  that  any  worse  contrivance  has  yet  been 
discovered,  as  far  as  power  economy  alone  is  concerned, 
than  the  common  direct-acting  steam-pump.  There  are 
enormous  clearances  to  be  filled  at  every  stroke  of  the 
pump,  without  any  compensation  for  the  waste  or  any  justi- 
fication of  its  existence,  except  the  very  insistent  one  that 
the  clearance  is  one  of  the  conditions  necessary  to  make 
the  pump  go,  or  is  necessary  when  tired  steam  is  used. 
Not  the  slightest  advantage  can  be  taken  of  the  possible 
expansion  of  the  steam  or  air.  Too  often  the  reverse  of 
expansion  occurs,  and  at  the  termination  of  the  stroke  the 
cylinder  is  filled  to  a  higher  pressure  than  was  required  to 


1/2  COMPRESSED   AIR. 

make  the  stroke,  and  all  the  cylinderful  to  be  immediately 
exhausted.  This  may  be  worse  with  a  duplex  than  with  a 
single  pump,  as  either  piston  may  reach  the  end  of  its 
stroke  and  the  cylinder  may  then  be  overfilled  with  steam  or 
air  while  it  is  waiting  to  have  its  valve  reversed  by  the  move- 
ment of  the  other  piston.  But  it  always  happens,  in  addi- 
tion to  this,  that  the  pumps  are  not  proportioned  to  the 
work  to  be  done,  or  that  the  several  pumps  are  not  all  so 
proportioned  that  the  same  pressure  will  operate  each  of 
them.  Where  the  pumps  were  driven  by  steam  trans- 
mitted a  long  distance,  it  might  have  been  well  to  calculate 
upon  lower  steam  as  the  distance  increased,  and  to  plan  the 
pumps  accordingly,  but  there  is  no  calculation  of  the  kind 
undertaken.  The  pumps  are  simply  bought  ready-made, 
and  they  fit  about  as  well  as  other  ready-made  goods, 
Take  the  published  list  of  the  pumps  in  the  mine  that  we 
are  talking  about.  The  actual  heads  under  which  the  sev- 
eral pumps  are  operated  is  given,  and  I  have  added  10  per 
cent  to  the  pressures  due  to  those  heads,  and  the  operating 
pressures  required  in  the  steam-cylinders  of  the  several 
pumps  are  then  as  follows  : 

No.     12345678 
Lbs.  32         28         10        31         39         23        37         32 

No.  3  is  a  small  pump  and  runs  bu,,  a  short  time  each 
day,  and  is  not  of  much  account  to  us.  Nos.  4,  5,  and  6, 
however,  are  the  largest  pumps  employed,  located  near 
each  other,  and  delivering  under  the  same  head.  If  the 
above  are  not  the  several  pressures  required,  they  must  be 
nearly  in  these  ratios,  and  they  seem  to  complacently  ig- 
nore the  fact  that  all  the  pumps  should  be  operated  by 
approximately  the  same  pressure,  especially  if  compressed 


COMPRESSED   AIR  FOR  PUMPING.  1/3 

air  is  used  to  drive  them.  The  pump  for  which  the  lowest 
pressure  is  sufficient,  running  under  throttle,  is  quite  likely 
to  fill  its  cylinder  with  air  at  the  highest  pressure  before  the 
exhaust  occurs.  Unless  the  piping  is  outrageously  inade- 
quate the  air  pressure  will  be  practically  the  same  through- 
out the  mine.  In  this  mine  the  pipe  capacity  is  liberal, 
and  it  is  safe  to  assume  that  the  air  pressure  in  the  pipes 
supplying  these  eight  pumps  will  not  be  found  to  vary  more 
than  i  Ib.  or  2  Ibs.  at  the  most  throughout  the  series. 

At  this  writing,  the  air-plant  having  been  in  operation 
considerably  over  a  year,  I  have  the  written  word  of  the 
superintendent  that  its  efficiency  has  not  been  to  this  day 
definitely  ascertained.  All  of  the  pumps  have  not  been 
operated  at  once  except  in  emergencies.  The  most  defi- 
nite statement  obtainable  is  that  with  the  compressors  at  a 
certain  speed  six  of  the  eight  pumps  are  run  at  once,  which 
tells  us  nothing,  as  we  do  not  know  the  speed  of  the  pumps. 
It  would  be  a  simple  matter,  when  everything  was  going, 
and  at  any  time  agreed  upon,  to  station  a  man  at  each 
pump  and  let  him  count  the  strokes,  and  this,  in  connec- 
tion with  the  revolutions  of  the  compressors,  would  tell  us 
much.  From  a  knowledge  of  all  the  available  data  in  this 
case,  and  from  some  knowledge  of  similar  cases,  I  am  will- 
ing to  hazard  the  assertion  that  not  more  than  20  per  cent 
of  the  I.H.-P.  at  the  steam-cylinders  of  the  compressors  is 
to  be  found  in  the  weight  of  water  delivered  by  the  pumps. 

Of  course  the  arrangement  as  it  stands  is  a  great  improve- 
ment over  the  use  of  direct  steam  at  the  pumps,  and  every- 
body is  to  be  congratulated  over  the  change.  Not  only 
is  there  an  actual  reduction  in  the  total  consumption  of 
steam,  where  it  is  used  in  the  cylinders  of  the  air-compres- 
sors instead  of  in  the  cylinders  of  the  pumps,  but  all  of  the 


174  COMPRESSED    AIR. 

annoyance,  delayj  danger,  and  expense  of  the  steam-distri- 
bution is  avoided.  That,  with  suitable  pumps,  a  different 
air  pressure  in  the  pipes,  and  a  consistent  combination  of 
machinery  throughout,  the  same  work  could  be  done  for 
one  half  of  the  fuel  is  not  worth  considering,  for  this  is  a 
coal  mine,  you  know.  But  if  the  same  work  could  have 
been  done  with  one  half  of  the  boiler  and  compressor- 
plant,  that  would  been  worth  considering  even  at  a  coa! 
mine,  and  is  deserving  of  much  serious  thought  where  the 
installation  of  additional  plants  is  under  consideration. 

Something  may  of  course  be  said  upon  the  other  side  of 
this  case,  and  toward  shifting  the  responsibility  for  it. 
Mine  managers  are  nof  pump  experts.  The  attitude  of  the 
pump  builders  is  similar  to  that  of  the  compressor  builders. 
They  simply  sell  the  pumps  and  they  know  little  about  how 
people  may  employ  them,  as  I  have  been  told  by  agents  and 
salesmen  of  pump  establishments.  No  special  pumps  are 
built  to  be  operated  by  compressed  air.  There  should  be 
such  pumps  in  the  market,  and  compressed  air  should  not 
be  used  with  any  thought  of  economy  in  the  common  direct- 
acting  steam-pump.  The  pump  should  be  a  geared  pump, 
and  the  air-motor  should  be  an  engine  with  a  cut-off 
adopted  to  the  pressure  of  air  employed. 

Right  here  seems  to  be  offered  a  fine  opportunity  for 
comparison  between  electricty  and  compressed  air  for 
power-transmission.  Pumping  is  a  line  of  work  that  either 
may  do,  and  with  little  apparent  unfair  advantage  in  the 
conditions.  The  same  pumps  that  are  being  put  in  to  be 
operated  by  electricity  would  be  equally  adapted  to  be  opera- 
ted by  compressed  air,  by  the  substitution  of  an  air-engine 
for  the  electric  motor  and  an  adjustment  of  the  gearing  to 
correspond,  and  a  fair  comparison  of  the  results  might  be 


COMPRESSED   AIR  FOR  PUMPING.  1 75 

made.  In  a  Western  town  a  pumping-plant  has  recently 
been  installed  to  be  driven  by  electricity.  The  pumps  arc 
bought  by  the  town,  and  the  local  electric-lighting  companj 
undertakes  to  maintain  and  operate  them,  transmitting  the 
current  2000  ft.,  for  4  cents  per  1000  gals,  delivered  against 
a  pressure  of  60  Ibs.,  which  is  about  twenty  times  the  fuel 
cost  for  the  same  work  in  the  best  pumping-engines  of  the 
day.  Electricity  might  sublet  this  contract  to  compressed 
air  for  one  quarter  of  the  figure,  and  the  air  would  be 
greatly  inflated  over  its  good  luck  in  getting  the  job. 

In  the  general  work  of  pumping  there  is  evidently  a  great 
field  still  to  be  occupied  by  compressed  air.  It  is  the 
natural  power-transmitter,  and  incomparably  the  best,  for 
mining  operations ;  and  as  the  mine  pumping  requires 
more  power  than  the  rock  drills,  more  compressed  air 
should  be  used  in  our  mines  for  the  pumping,  while,  as 
a  matter  of  fact,  probably  not  one  quarter  as  much  air  is 
used,  and  steam  or  mechanical  transmission  of  power  is 
employed  at  great  inconvenience  and  expense.  A  pressure 
of  6  atmospheres,  which  is  very  suitable  for  the  rock  drills, 
could  also  be  used  to  good  advantage  for  the  pumps,  and 
proper  arrangements  for  cooling  and  draining  the  air  would 
fully  dispel  all  danger  of  freezing,  which  is  the  prevalent 
bugbear  of  compressed-air  practice. 

Besides  the  pumping  for  mines  there  is  the  constantly 
recurring  problem  of  power-transmission  for  the  water- 
supply  of  towns  and  cities,  and  compressed  air  is  well 
adapted  for  such  service.  With  a  general  compressed-air 
service  established  in  our  large  cities  the  air  would  be  ready 
.to  operate  the  thousands  of  pumps,  now  driven  mostly  by 
isolated  hot-air  engines,  which  supply  the  tanks  upon  the 
roofs  of  high  buildings,  or  of  buildings  on  ground  too  high 
for  the  established  water-supply  to  reach. 


CHAPTER  XIX. 

A  LIST  OF  THE  VARIOUS  APPLICATIONS  OF 
COMPRESSED  AIR. 

THIS  list  is  intended  to  include  only  the  direct  applica- 
tions of  compressed  air  to  specific  uses,  and  not  its  employ- 
ment in  an  air-motor,  or  where  it  takes  the  place  or  does 
the'  work  of  a  steam-engine  or  other  power-developer.  The 
list  is  of  course  incomplete,  as  such  a  list  must  necessarily 
be,  for  the  applications  of  compressed  air  develop  faster 
than  they  can  become  generally  known  and  recorded.  A 
slight  explanation  of  the  way  in  which  the  air  is  used  is 
given  in  a  number  of  cases. 

Acids,  Raising  or  Transferring.  —  Compressed  air  is 
largely  used  for  this  purpose  in  chemical  works,  or  where 
acids  are  handled  in  bulk  or  in  large  quantities  and  where 
contact  with  the  metals  cannot  be  allowed.  Vessels  con- 
taining the  acid  are  subjected  to  a  pressure  of  air  inside 
them  and  above  the  acid,  and  the  pressure  of  the  air  causes 
the  acid  to  flow  wherever  the  pipe  may  lead  it. 

Accumulator  for  Hydraulic  Hoisting  Service. — The  com- 
pressed-air accumulator  takes  the  place  of  the  heavy  weights 
long  used  for  maintaining  a  uniform  and  constant  pressure 
and  regulating  the  supply  of  water  necessary  in  operating 
hydraulic  cranes,  lifts,  etc.  The  air,  compressed  to  the 
pressure  required  to  be  maintained,  is  contained  in  an  up- 
right cylindrical  vessel  of  considerable  capacity.  The  water 

176 


A    LIST  OF   THE    VARIOUS  APPLICATIONS.      IJJ 

rises  and  falls  in  the  lower  part  "of  the  vessel,  and  a  consider- 
able fluctuation  of  level  is  possible  without  great  change  in 
the  air  pressure.  A  float  upon  the  surface  of  the  water 
controls  the  movement  of  a  duplex  pump  to  maintain  the 
required  water-supply. 

Aerated  Bread. 

Aerated  Fuel. — A  jet  of  compressed  air  vaporizes  or 
atomizes  crude  petroleum  in  furnaces  which  have  a  wide  ap- 
plication in  the  arts  wherever  great  heat  with  perfect  control 
is  required,  as  in  glass  factories,  brick  or  lime  kilns,  forges 
and  metal  works,  etc.  The  system  is  also  used  for  gener- 
ating steam,  but  for  that  purpose  is  not  generally  cheaper 
than  coal.  Lamps  using  compressed  air  with  oil  in  a  similar 
way  are  much  used  for  out-of-door  work,  also  in  rolling- 
mills,  railroad  yards,  etc. 

Aerating  Molten  Metal  in  the  Bessemer  Process. — This 
was  a  revolutionary  application  of  compressed  air,  and  of 
untold  importance  in  the  manufacture  and  in  the  promotion 
of  the  use  of  steel.  The  air  is  forced  up  through  a  mass  of 
melted  cast  iron,  burning  out  the  carbon  and  in  a  few 
minutes  converting  the  entire  mass  into  steel,  thus  produc- 
ing steel  cheaper  than  iron. 

Aerating  Water. — The  aeration  of  water  is  usually  carried 
on  in  connection  with  its  filtration,  and  is  equally  necessary 
in  many  cases  to  render  the  water  wholesome  and  potable. 
Extensive  works  for  the  purpose  are  provided  in  connection 
with  the  water-supply  of  many  towns  and  cities.  The  air 
is  made  to  traverse  a  series  of  water-tanks,  passing  succes- 
sively up  through  the  contents  of  each,  carrying  off  volatile 
and  objectionable  constituents  and  imparting  the  necessary 
oxygen. 

Agitating  Syrups  in  Sugar-refineries. 


178  COMPRESSED    AIR. 

Air-brake. — The  air-brake,  in  use  upon  all  passenger 
trains,  and  also  largely  used  for  freight,  puts  the  control  of 
the  train  entirely  with  the  engineer.  Before  the  use  of 
compressed  air  for  this  purpose  the  brakes  upon  each  car 
were  applied  and  released  separately  by  individual  brake- 
men  upon  steam-whistle  signals  from  the  engineer.  The 
brakeman  is  discharged  and  the  whistle  is  seldom  heard. 
The  air  is  compressed  by  an  air-brake  pump  upon  the 
locomotive,  and  there  are  over  30,000  of  these  air-brake 
pumps  in  use,  a  number  greater,  perhaps,  than  that  of  all 
other  air-compressors  together.  The  air-brake  pump  has 
begotten  a  great  number  of  new  applications  of  compressed 
air,  especially  in  connection  with  the  different  departments 
of  railroad  service. 

Air-brake  upon  Street  Raihvays. — The  value  of  the  air- 
brake upon  the  steam-roads  and  the  necessity  for  a  quicker 
and  more  efficient  brake  for  street-cars,  now  that  the  cable 
and  the  trolley  have  made  them  heavier  and  have  increased 
their  speed,  are  leading  rapidly  to  the  adoption  of  the  air- 
brake for  street-railway  service.  They  air-compressing 
pump  on  the  street-car  is  operated  by  a  crank  or  eccentric 
upon  one  of  the  axles  of  the  car. 

Air,  Dense,  see  Dense-air  Refrigerating. 

Air-hoist. — This  term  is  used  in  contradistinction  to  the 
pneumatic  crane,  the  crane  generally  employing  drums  and 
gearing  or  other  complicated  mechanism,  while  the  move- 
ment of  the  hoist  is  simple,  direct,  and  of  limited  range. 

Air-jack. — Air-jacks  are  largely  used  in  railroad  shops 
and  are  a  distinct  outgrowth  of  the  air-brake  purnp,  the 
pump  being  always  at  hand  or  easily  procurable  with  other 
railroad  supplies,  and  may  be  readily  piped  up  wherever  it 
may  be  wanted.  The  jacks  are  more  properly  air-lifts,  usu- 


A    L/S7*  OF   THE    VARIOUS  APPLICATIONS.     1/9 

ally  operating  from  below  the  load  to  be  lifted.  Many 
jacks  are  sunk  in  specially  prepared  pits  under  repair  tracks 
for  taking  out  wheels  and  axles.  Portable  jacks  are  in  use 
which  have  wheels  and  handles  like  a  barrel  truck.  Stand- 
ing the  truck  up  sets  the  lifting  cylinder  upon  its  base,  and 
the  jack  is  "at  once  ready  for  work.  The  air  pressure  is 
supplied  by  an  air-hose  connected  with  a  convenient  branch 
upon  the  air-supply  pipe.  "  Pulling  down  "  jacks  are  made 
for  pulling  down  defective  sills  upon  freight  cars. 

Air-lift  Pump. — The  Pohle  air-lift  pump  is  not  prop- 
erly a  pump,  except  that  it  is  employed  for  raising  water, 
and  it  has  no  working  or  moving  parts  of  any  kind.  A 
vertical  water-pipe,  usually  in  a  bored  or  artesian  well,  ex- 
tends down  some  distance  below  the  level  of  the  water  to 
be  lifted,  and  at  the  lower  end  of  it,  which  is  open,  a  com- 
pressed-air pipe  discharges  the  air  upward  into  the  column 
of  water,  and  the  mingled  air  and  water  rise  and  flow  from 
the  upper  end  of  the  water-pipe.  The  flow  of  water  con- 
tinues as  long  as  the  air  is  supplied.  The  pump  gives  ex- 
cellent economical  results  and  is  highly  commended. 

Air-lifts,  see  Elevators. 

Air-lock  Doors. — The  air-lock  is  used  for  the  ingress  and 
egrees  of  workmen  and  material  to  and  from  caissons,  the 
excavating  chambers  of  soft-ground  tunnels,  or  wherever 
operations  are  carried  on  under  air  pressure.  The  lock  is  a 
chamber  of  sufficient  size  to  receive  two  or  more  men  at  a 
time  or  a  bucket  of  material.  It  is  provided  with  two  sets 
of  doors  and  valves  to  admit  or  discharge  the  air.  To 
enter  the  working  chamber  the  outer  door  of-  the  lock 
is  opened,  then  the  men  enter  the  lock  and  this  door 
is  closed.  A  valve  is  opened  admitting  air  from  the 
working  chamber  until  an  equal  pressure  is  attained  in 


180  COMPRESSED   AIR. 

the  lock,  when  the  inner  door  may  be  opened  and  the  men 
admitted.  The  same  operation  occurs  in  the  admission 
of  material,  and  the  process  is  reversed  for  egress.  By  a 
late  improvement  the  outer  doors  of  the  air-lock  are  opened 
and  closed  by  the  pressure  of  the  air  acting  upon  pistons 
connected  with  the  doors,  and  the  locks  are  operated  more 
rapidly  than  formerly,  especially  for  the  hoisting  or  lowering 
of  material. 

Applying  Hose-couplings. 

Armor,  Diving,  see  Diving-armor. 

Asphalt-refining,  see  Refining  Asphalt. 

Automatic  Pump,  see  Ejector. 

Automatic  Fire-extinguisher. 

Balloon,  Water,  see  Raising  Ships. 

Beating  Eggs. 

Beer-pump. — The  use  of  compressed  air  for  forcing  beer 
from  barrels,  for  the  retail  trade  in  saloons  and  hotels,  must 
be  more  extensive  than  its  use  for  the  air-brake.  The  air 
pressure  for  this  service  is  either  provided  by  hand  power 
or  automatically  by  hydrant  pressure. 

Bell-ringing,  see  Ringing  Bells. 

Bellmvs,  Organ,  see  Organ  Bellows. 

Blacksmith's  Fires. — Where  a  compressed-air  supply  is 
maintained  for  operating  rock  drills  or  general  machinery, 
and  at  a  pressure  of  6  or  7  atmospheres,  the  air  for  blow- 
ing the  blacksmith's  fire  is  sometimes  taken  from  the  com- 
pressed-air pipes.  This  is  a  costly  way  of  supplying  air 
at  such  a  light  pressure.  The  power  required  for  com- 
pressing each  cubic  foot  of  free  air  for  the  rock  drill  is 
probably  ten  times  as  great  as  would  be  required  for  the 
blowing  pressure. 

Blast,  Sand,  see  Sand-blast. 


A    LIST  OF   THE    VARIOUS  APPLICATIONS.      l8l 

Block  Signal,  see  Switch  and  Signal  Service. 

Bessemer  Process,  see  Aerating  Molten  Metal. 

Boiler-shop. — Compressed  air  is  now  capable  of  supply- 
ing all  the  power  required  for  operating  boiler-shops,  ma- 
chines or  apparatus,  mostly  portable,  being  provided  for  all 
of  the  operations  involved.  Hoisting,  punching,  shearing, 
drilling,  tapping,  reaming,  riveting,  chipping,  caulking,  and 
'screwing  in  and  cutting  off  stay-bolts  are  all  quickly,  effi- 
ciently, and  economically  done  by  compressed  air. 

Brake,  Air,  see  Air-brake. 

Bridge-building. — Compressed  air  is  a  valuable  assistant 
in  bridge-building,  both  in  the  preparation  of  the  material 
in  the  shop  and  in  the  erection  of  the  structure.  In  the 
shop  the  air  is  used  for  hoisting  and  in  portable  tools  for 
drilling,  reaming,  riveting,  chipping,  etc.,  as  in  the  boiler- 
shop.  The  erection  of  the  bridge  would  be  in  many  cases 
impossible  without  the  compressed  air  in  the  caissons  for 
the  piers,  while  in  the  work  of  erection  the  portable  tools 
are  called  in  again. 

Caisson. — The  caisson  is  used  principally  for  excavations 
under  water,  and  subsequently  for  building,  in  place  of  the 
material  removed,  bridge  piers,  or  solid  masonry  for  any. 
purpose.  The  caisson  is  essentially  an  open  box,  of  a  shape 
corresponding  to  the  purpose  desired,  closed  at  the  top  and 
loaded  above  until  it  sinks  where  the  work  is  to  be  done. 
The  caisson  being  open  at  the  bottom,  the  water  is  excluded 
by  the  maintenance  of  an  air  pressure  within,  the  pressure 
increasing  with  the  submergence  until  at  a  depth  of  80  or 
100  feet  the  limit  of  human  endurance  is  reached.  Men  and 
material  pass  into  and  out  of  the  caisson  by  means  of  the 
air-lock.  The  men  within  the  caisson  remove  the  material 
with  which  the  lower  edge  of  the  caisson  comes  in  contact 


1 82  COMPRESSED    AIR. 

until  a  satisfactory  foundation  is  reached,  and  the  caisson 
is  then  built  up  full  of  substantial  masonry  and  allowed  to 
remain  as  a  part  of  the  permanent  structure,  which  is  con- 
tinued above  it  to  any  height  desired.  The  caisson  is  now 
also  frequently  used  in  obtaining  suitable  foundations  and 
supports  for  the  tall  and  heavy  office  buildings  erected  in 
our  large  cities. 

Caissons,  Expelling  Soft  Material  from. — This  is  a  use  of 
compressed  air  entirely  distinct  from  its  primal  function  in 
the  caisson  of  excluding  the  water  so  that  the  men  may  be 
able  to  work  in  it.  When  any  soft  material  is  found  in  the 
progress  of  the  excavation,  it  is  now  customary  to  expel  it 
through  a  pipe  carried  up  through  the  top  or  side  of  the 
caisson,  the  pressure  of  the  air  within  supplying  all  the  power 
required.  The  pipe  is  provided  with  a  quick-closing  valve, 
so  that  when  the  material  has  all  run  out  the  air  may  not 
escape. 

Caissons,  Operating  Air-lock  Doors,  see  Air-lock  Doors. 

Cars,  Propelling,  on  Street  Raihvays. — Compressed  air  is 
not  yet  extensively  used  for  this  purpose  in  the  United 
States,  but  is  permanently  established  and  successful  in 
Paris  and  elsewhere  in  Europe.  Experimental  cars  in  the 
United  States  show  excellent  results,  and  the  general  adop- 
tion of  the  system  in  the  near  future  is  more  than  probable. 
In  cost  of  plant,  in  facility  of  introduction,  in  economy  of 
operation,  and  in  the  entire  absence  of  objectionable  feat- 
ures the  compressed-air  system  surpasses  all  others.  The 
principal  delay  as  to  its  extensive  introduction  is  in  deter- 
mining the  ultimately  best  of  many  details  of  construction 
and  operation. 

Cars,  Dumping. — Cars  dumped  by  compressed  air  are 
used  in  handling  earthwork  in  railroad  construction  and 


A    LIST   OF   THE    VARIOUS  APPLICATIONS.      183 

similar  service,  also  for  coal,  ore,  limestone,  etc.  An  air- 
cylinder  and  piston  under  the  car  dumps  the  load  upon 
either  side  as  may  be  desired.  An  entire  working  train 
may  be  dumped  at  once,  or  a  man  may  dump  each  car 
separately. 

Cars,  Loading. — One  of  the  functions  of  the  air-hoist  or 
of  the  pneumatic  crane. 

Cars,  Unloading. — Unloading  by  lifting  the  load  from 
the  car  by  the  air-hoist  instead  of  by  dumping.  Oil- tank 
cars  are  discharged  to  a  higher  level  by  air  pressure  ad- 
mitted to  the  tank. 

Car  Roofs,  Sanding,  see  Sanding  Car  Roofs. 

Cars,  Cleaning. — This  system  is  now  generally  used  at 
railroad  termini.  A  supply  of  compressed  air  is  main- 
tained, and  a  hose  is  led  into  the  car  or  coach  to  be  cleaned, 
with  a  nozzle  for  discharging  the  air  and  a  cock  for  regu- 
lating or  shutting  it  off.  The  jet  of  air  is  successively 
passed  over  the  various  parts  of  the  interior  of  the  car  and 
the  dust  and  other  loose  material  is  driven  off  at  once. 

Car  Seats,  Cleaning. — The  seats  and  cushions,  rugs,  etc. 
are  removed  from  the  car  and,  supported  upon  wooden 
horses,  are  thoroughly  and  quickly  cleaned  by  the  air  jet. 

Car  Sills,  Pulling  Down,  see  Pulling  Down  Jacks. 

Car  Wheels  and  Axles,  Removing,  see  Removing  Car 
Wheels  and  Axles. 

Carriages,  Gun,  see  Gun-carriages. 

Carpets,  Cleaning. — The  patented  arrangement  of  the 
writer  consists  of  a  grated  or  perforated  level  floor  or  an 
inclined  or  vertical  surface  upon  or  against  which  the 
carpet  to  be  cleaned  is  spread.  The  carpet  is  then  tra- 
versed by  an  air-delivery  pipe  upon  wheels  and  with  handles 
like  a  lawn-mower.  A  hose  conveys  the  air  to  the  delivery- 


1 84  COMPRESSED    AIR. 

pipe  and  it  emerges  in  a  series  of  fine  jets  close  to  tne  sur- 
face of  the  carpet,  rapidly  expelling  the  dust  and  dirt.  An 
exhaust  fan  draws  the  dust  away  whether  liberated  above 
or  below  the  carpet. 

Castings,  Chipping. — One  of  the  adaptations  of  the  pneu- 
matic tool,  which  see. 

Caulking.— Now  generally  done  by  compressed  air,  espe- 
cially in  boiler-  and  tank-work  and  upon  the  seams  and 
joints  of  steel  ships.  This  is  another  of  the  uses  of  the 
pneumatic  tool. 

Cash-carriers. — Generally  used  in  the  large  retail  stores. 

Canal  Locks  or  Lifts. — An  important  invention,  lifting 
vessels  to  any  height  by  a  single  lift,  one  air-lift  taking  the 
place  of  several  of  the  old  style  of  locks.  As  the  lift  is 
balanced,  but  little  power  is  required  to  operate  and  little 
water  is  lost. 

Channelling-machines. — A  modification  of  or  more  elabo- 
rate application  of  the  rock  drill,  applied  either  to  getting 
out  stone  of  required  shape  and  dimensions  from  its  native 
bed,  or  cutting  smooth  channels  in  the  solid  rock,  as  at  the 
Chicago  Drainage  Canal. 

Chemical  Works. — In  chemical  works  a  supply  of  com- 
pressed air  is  constantly  maintained  and  .employed  for 
various  uses,  such  as  the  pneumatic  pump  or  ejector,  the 
air-lift  and  aerating  processes. 

Cleaning. — Compressed  air  is  employed  in  cleaning  vari- 
ous things,  such  as  flues,  carpets,  castings,  by  widely  dif- 
ferent apparatus  and  processes. 

Chipping. — Another  of  the  applications  of  the  pneumatic 
tool,  especially  used  in  boiler-work,  bridge-  and  ship-work, 
structural  ironwork,  foundries,  etc. 

Clipping  Horses. 


A    LIST  OF   THE    VARIOUS  APPLICATIONS.      185 

Clocks,  Operating. — In  extensive  use  in  Paris.  Almost 
the  only  service  rendered  by  compressed  air  that  could  be 
done  as  well  or  better  by  electricity. 

Coal  Drills,  Operating. — These  are  revolving  drills  or 
augers  boring  holes  very  rapidly  for  light  charges  of  ex- 
plosives. 

Coal-mining  Machines. — The  cutting  tool  of  the  ma- 
chine reciprocates  like  a  rock  drill.  It  is  mounted  upon 
wheels  and  cuts  under  the  seam  of  coal  to  a  horizontal 
depth  of  five  or  six  feet,  when  the  coal  may  be  broken 
down  and  removed. 

Coal  or  Culm  Conveyors. 

Colors,  Spraying.— Used  in  silk  factories  for  spraying 
colors  upon  silk  or  satin  ribbons.  A  recently  perfected 
process  sprays  colors  upon  pottery,  sometimes  in  liquid 
form  and  sometimes  as  a  powder.  Varied  and  novel  effects 
are  produced  by  applying  several  colors  simultaneously  by 
separate  jets,  or  color  and  glazing  may  be  mixed  and 
sprayed  together. 

Conductor's  Train  Signal. — Extensively  used  upon  the 
best  railroads. 

Cooling. — This  is  one  of  the  general  and  widely  appli- 
cable uses  of  compressed  air.  The  fall  of  temperature  in 
compressed  air  upon  release  is  used  for  cooling  drinking- 
water,  for  cooling  houses  or  apartments,  theatres,  for  gen- 
eral refrigeration,  ice  factories,  and  cold-storage  warehouses. 

Copying-presses. 

Couplings,  Applying,  to  Hose. 

Cranes. — Swinging,  jib,  or  travelling  cranes. 

Crossings,  Gates  at,  see  Gates  at  Railroad  Crossings. 

Cupolas,  Raising  Stock  to. 

Cutting  off  Stay-bolts. — This  is  done  by  a  special  style  of 


lS6  COMPRESSED   AIR. 

portable  shears,  requiring  no  skilled  labor,  and  does  not 
loosen  the  stay-bolt. 

Cuts  and  Quar  ries,  Driving  Machinery  in. 

Dampening  Laundry-work. — The  spraying-jet  takes  the 
place  of  the  Chinaman's  mouth,  said  to  be  employed  for  the 
same  purpose. 

Dense-air  Refrigerating  Process. — Used  upon  warships 
and  elsewhere.  The  same  air  is  used  over  and  over,  com- 
pressed to  say  15  atmospheres  and  expanded  to  say  5  atmos- 
pheres, and  the  same  refrigerative  effect  is  accomplished 
by  less  power  and  in  smaller  compass  than  when  lower 
pressures  are  employed. 

Direct-acting  Hoist. — Quicker  and  simpler  than  any  other. 

Disappearing  Gun-carriage. 

Disposal  of  Sewage. — The  Shone  ejector  automatically 
raises  the  sewage  to  give  it  head  to  flow  where  the  requisite 
grade  cannot  be  maintained  in  the  sewer. 

Distributing  Sand  on  Locomotives. — Advantage  is  taken  of 
the  air-supply  for  the  brakes,  and  the  tracks  are  sanded, 
giving  better  adhesion  and  with  less  waste  of  sand. 

Diving-bell. 

Diving-armor. 

Doors,  Furnace,  Raising  and  Lowering. 

Doors,  Air-lock  Doors,  Operating. 

Doors,  Opening,  in  Offices  and  Residences. — This  is  a  sug- 
gested rather  than  an  accomplished  use  of  compressed  air, 
but  perfectly  feasible  where  the  air-supply  exists.  As  we 
now  automatically  close  our  doors,  so  may  we  open  them  in 
welcome  when  any  one  approaches. 

Drainage  Systems. — Compressed  air  is  variously  employed 
in  such  service  the  conditions  determining  the  arrangement. 

Dredging. 


A    LIST  OF  THE    VARIOUS  APPLICATIONS.     l8/ 

Drills. — Revolving  drills  of  various  kinds,  portable  drills, 
metal  drills,  coal  drills,  diamond  drills  for  prospecting. 

Drills,  Rock. — Reciprocating  or  percussion  drills.  The 
use  of  compressed  air  employing  more  of  it  than  any  other. 

Drinking-  water  y  Cooling. 

Drinking-water,  Aerating. 

Driving  Stay-bolt  Tops. — One  of  the  special  uses  of  com- 
pressed air  in  the  boiler-shop,  simple,  but  saving  much  time 
and  labor. 

Driving  Machinery  in  Shops. — Two  or  three  shops  use 
compressed  air  for  driving  all  their  tools,  dispensing  with  all 
shafting  except  a  light  line  for  a  group  of  small  tools. 

Driving  Pumps. — An  undeveloped  use  of  compressed  air 
of  great  importance  and  promise,  see  Chapter  XVI. 

Driving  Motors  or  Air-engines. 

Drop  Pits. — This  is  the  technical  name  for  an  arrange- 
ment in  use  in  railroad  repair  shops.  The  car  is  run  over 
the  pit  and  an  air-jack  or  hoist  lowers  wheels  and  axles  to 
be  removed  or  hoists  new  ones  in  place. 

Droppers  for  Cattle. — This  name  is  given  to  one  series  of 
air-hoists  used  in  the  Armour  packing-house  and  similar 
establishments.  The  bullock,  suspended  by  the  heels,  after 
bleeding  and  decapitation  is  conveyed  by  a  continuously 
travelling  overhead  railway  to  a  hook  on  one  of  the  drop- 
pers, the  hook  being  held  up  by  the  pressure  of  the  air  with 
sufficient  force  to  sustain  the  weight.  Upon  releasing  the 
air  the  bullock  is  dropped  upon  the  floor  for  skinning,  dis- 
embowelling and  such  interesting  operations. 

Drop  Weight  for  Breaking  Castings,  Lifting. — A  popular 
use  of  compressed  air  in  the  yards  of  foundries  that  are 
fully  equipped  with  it.  A  single  hoisting  cylinder  is  used 


1 88  COMPRESSED   AIR. 

with  multiplying  sheaves  so  that  the  hoist  of  the  weight  is 
usually  six  or  eight  times  the  travel  of  the  piston. 

Dry  Dock. — The  compressed-air  dry  dock  may  have  all 
the  advantages  of  the  independent  floating  dock  at  less  first 
cost  and  less  cost  of  operation  and  maintenance. 

Dumping  Cars. 

Dynamite  Gun. — The  only  safe  way  yet  devised  for 
throwing  the  high  explosives.  Decided  to  be  of  value  for 
coast  defence.  A  number  of  these  guns  now  under  con- 
struction. 

Eggs,  Beating. 

Elevators. — Compressed  air  adapts  itself  readily  to  all  the 
various  demands  of  elevator  service,  and  is  used  for  passen- 
gers or  freight,  by  direct  or  multiple  lift  of  a  single  piston, 
or  with  an  air-motor  with  gearing  and  drums. 

Elevators,  Coal  and  Culm. 

Elevators,  Indicators  on. — Indicators  operated  by  com- 
pressed air  to  signal  or  to  inform  the  passenger  and  the 
operator. 

Ejector. — Used  for  automatically  transferring  sewage  or 
other  liquids.  The  air  pressure  being  maintained,  the 
chamber  is  alternately  filled  by  the  flow  of  the  liquid,  and 
emptied  by  its  ejection  or  expulsion  to  a  higher  level. 

Engines,  Fire,  see  Fire-engines. 

Engine  Works,  Driving. 

Expelling  Soft  Material  from  Caissons. 

Extinguishers,  Automatic  Fire. 

Factories,  General  Use  in. — A  great  and  rapidly  increas- 
ing number  of  factories  are  equipped  with  compressed  air, 
first  of  all  for  direct  hoisting,  and  subsequently  for  various 
other  purposes. 

Finish,  Satin,  on  Metal-work,  see  Satin  Finish. 


A    LIST  OF   THE    VARIOUS  APPLICATIONS.     189 

Filtering  Water. 

Fire-engines. — A  suggested  use  of  compressed  air,  per- 
fectly feasible  wherever  a  general  supply  of  compressed  air 
is  distributed. 

Fire-extinguisher. 

Fires,  Blacksmiths',  Blowing. 

Fires,  Kindling. — The  aerated  oil  jet  is  used  in  many 
round-houses  for  starting  the  fires  in  the  locomotives  at 
one  tenth  of  the  cost  of  wood  kindlings. 

Flues,  Cleaning. 

Forcing  Oil. — Transferring  oil  from  tanks  to  barrels,  or 
vice  versa. 

Foundry,  General  Service  in. 

Fountains,  Cooling,  see  Chapter  XIII. 

Furnace  Doors,  Raising  and  Lowering. 

Gas,  Aerating. 

Gates  at  Crossings,  Operating. — In  connection  with  the 
compressed-air  switch  and  signal  service. 

Gear  Steering,  on  Ships. 

General  Hoisting  Service. — Some  establishments  employ- 
ing more  than  a  hundred  air-hoists. 

Glass  Factories. 

Glass-blowing. 

Grain  Elevators. 

Granite-carving. 

Granite-cutting. — One  of  the  uses  of  the  pneumatic  tool 

Grates,  Shaking. 

Gun-carriage,  Disappearing. 

Gun,  Pneumatic.— Valuable  for  coast  defence,  throwing 
high  explosives. 

Guns,  Sporting  or  Target. 

Hammer,  Pneumatic. 


IQO  COMPRESSED   AIR. 

Hardle  Car-motor. 

Hoisting  Cattle,  Beef. — Air-hoists  used  exclusively  in  the 
largest  packing-houses. 

Hoist,  Direct-acting  Vertical-cylinder. 

Hoist,  Geared. 

Horses,  Clipping. 

Hose'couplings,  Applying. — The  machine  for  this  purpose 
is  said  to  have  paid  for  itself  in  one  day's  application  of  it. 

Hydraulic  Cranes. — Air  is  used  to  give  pressure  to  the 
water,  while  the  water  actually  does  the  hoisting  in  some 
lines  of  service  where  the  elasticity  of  the  air  would  be 
objectionable. 

Hydraulic  Pressure  Relief. — In  wood-pulp  machines  in 
paper-mills  a  hydraulic  feed  is  employed  which  is  some- 
times too  positive,  and  a  chamber  of  compressed  air  is  pro- 
vided to  relieve  it  and  prevent  breakage. 

Ice-making. 

Indicators  on  Elevators. 

Iron,  Drills  for. 

Iron  Furnaces,  Tapping. 

Iron  Bridge  Work. 

Ironwork,  Structural. 

Jacks,  Portable. 

Jacks,  "  Pulling  Down." 

Kindling  Fires  in  Locomotives. 

Lamps. — Aerated  oil  lamps  for  street  work,  railroad  op- 
erations, etc. 

Lard- re  fining. 

Laundry-work,  Dampening. 

Lifting  Drop  Weight  in  Foundry  Yards. 

Locks,  Canal. 

Lock  Doors  in  Caissons,  Operating. 


A   LIST  OF  THE    VARIOUS  APPLICATIONS.     IQI 

Loading  Cars. 

Locomotives  in  Mines,  Street  Railways,  etc* 

Locomotives,  Kindling  Fires  in. 

Medical  Preparations,  Spraying. 

Mekarski  System  of  Car  Propulsion. 

Mining  Coal. — Compressed  air  is  variously  used  in  coal 
mines,  for  "  coal-cutters,"  coal  augers,  rock  drills,  pumps, 
hoists,  etc. 

Mixing  Nitroglycerine. 

Moulding-machines. — In  the  foundry  compressed  air  rams 
or  presses  the  sand  in  the  moulding-machine,  lifts  the 
roould,  draws  the  pattern,  etc. 

Nitroglycerine,  Mixing. 

Operating  Air-drills  and  Punches 

Opening  Doors. 

Packages,  Transmitting. 

Painting. 

Pile-driver. 

Pits,  Drop,  see  Drop  Pits. 

Physicians'  Spraying  Apparatus. 

Pneumatic  Ejector. 

Pneumatic  Press. 

Pneumatic  Signal  for  Railway  Trains. 

Pneumatic  Tool. 

Pneumatic  Tubes  for  Transmission. 

Portable  Drill. 

Portable  Jack. 

Preserving  Timber,  the  Wood  Vulcanizing  Process,  which 
see. 

Press,  Copying. 

Press,  Straightening. 

Pottery,  Spraying  with  Colors. 


192  COMPRESSED   AIR. 

Process,  Bessemer,  see  Aerating  MoHen  Metal. 

Pulling  Down  Jacks,  see  Jacks,  Pulling  Down. 

Pump,  Air- Lift. 

Pump,  Automatic. 

Pump,  Beer. 

Pumps,  Operating. 

Pumping  Acids. 

Punch,  Portable. 

Punching  in  Boiler-shops,  etc. 

Quarries,  General  Work  in. 

Railways,  Street. 

Railroad  Shops,  Various  Uses  in. 

Railroad  Shops  and  Sheds,  Whitewashing. 

Raising  Stock  to  Cupolas  in  Foundries. 

Raising  Ships. — Air-tight  bags  are  attached  all  around  a 
sunken  ship,  or  placed  by  divers  in  the  hold,  then  inflated 
by  compressed  air,  and,  acting  like  balloons  in  the  air, 
when  their  combined  displacement  is  sufficient  the  ship 
rises. 

Refining  Lard. 

Refining  Asphalt. 

Refrigerating. 

Removing  Mandrels. 

Removing  Scale  from  Steel  Plates — another  of  the  uses 
of  the  pneumatic  tool. 

Ribbons,  Spraying  with  Colors. 

Ringing  Bells  on  Locomotives. 

Riveting. 

Rock  Drills,  Operating. 

Rock  Tunnels,  Driving,  All  Operations  in. 

Sand-blast. 


A    LIST  OF   THE    VARIOUS  APPLICATIONS.     193 

Sanding  Tracks.— Giving  better  distribution,  better  adhe- 
sion, and  wasting  less  sand  than  where  delivered  by  gravity. 

Sanding  Car  Roofs. — A  process  used  in  the  car-building 
or  repair  shops  in  connection  with  the  painting  of  the  roofs 
of  freight  cars.  The  sand  is  delivered  by  the  air  with  force, 
so  that  it  embeds  itself  in  the  paint,  forming  a  protection 
for  the  surface.  What  is  not  held  by  the  paint  is  removed 
by  the  same  blast  of  air  that  delivers  the  sand. 

Satin  Finish  on  Metals. — Used  on  plated  work  for  rail- 
road cars. 

Scale,  Removing,  from  Steel  Plates* 

Seats,  Cleaning. 

Sewage  Disposal. 

Sheathing  Pile-driver. 

Sheep  •  shearing. 

Ships,  Raising. 

Ships,  Steering. 

Ships,  General  Service  on. 

Shops,  Driving  Machine  Tools  in. 

Signal,  Block. 

Signal,  Conductor's. 

Silk  Manufacture. 

Silk  Ribbons,  Spraying, 

Skates. 

Soft-ground  Tunnels. 

Spraying  Laundry-work. 

Spraying  Colors  on  Silk  Ribbons. 

Spraying  Colors  on  Pottery. 

Stay-bolts,  Cutting  off. 

Stay  bolt  Taps,  Driving. 

Steering-gear  on  Ships. 

Stone-cutting. 


194  COMPRESSED    AIR. 

Storage,  Cold. 

Steel  Plates,  Removing  Scale  from. 

Street  Railways. 

Structural  Ironwork. 

Switches  and  Signals  on  Railroads. 

Syrups,  Agitating. 

Taps,  Driving,  in  Boiler-shops. 

Tapping  Iron  furnaces. 

Testing  brakes. 

Timber,  Preserving. 

Tires  for  Vehicles. 

Tool,  Pneumatic. 

Torpedo  Service. 

Tracks,  Sanding. 

Train  Signal. 

Transferring  Oil  or  Acids. 

Transmitting  Packages. 

Transmitting  Power  from  Waterfalls. 

Travelling-crane. 

Trucks,  Dumping. 

Tunnels,  Soft-ground. 

Tunnels  in  Rock. 

Turrets,  Operating  on  Warships. 

Unloading  Cars. — This  is  done  either  by  hoisting,  by 
dumping,  or  in  tank  cars  by  pressure  upon  the  service  of 
the  liquid. 

Ventilating. 

Vertical  Direct  Hoist. 

Vehicle  Wheel-tires. 

Warfare,  General  use  in. 

Water-balloon,  see  Raising  Ships. 

Water,  Raising. 


A    LIST  OF  THE    VARIOUS  APPLICATIONS.     1 95 

Water,  Aerating. 

Water,  Filtering. 

Wheels  and  Axles,  Hoisting  or  Removing. 

Whitewashing. 

Working  Turrets. 

World's  Fair  Painting. 

Wood  Vulcanizing. 

Works,  Chemical,  General  use  in. 


INDEX. 


Absolute  temperature,  n. 

Absolutely  isothermal  or  adiabatic  compression  impossible,  22. 

Accommodating  attitude  of  air,  138. 

Action  of  air  in  passages  of  two-stage  compressor,  75. 

Actual  curve  in  expansion  always  above  theoretical  adiabatic,  102. 

Actual  volume  of  air  the  basis  in  transmission  computations,  no. 

Additional  lines  on  indicator-diagram,  43. 

Adiabatic  and  isothermal  curves  not  required  for  computations,  51. 

Adiabatic  compression,  15. 

Adiabatic  curves  to  draw  on  diagram,  49. 

Advances  in  steam  economy,  38. 

Air-brake  pump,  128. 

Air-compression  line  simpler  than  the  steam-expansion  line,  49. 

Air-compressor  as  an  air-meter,  54. 

Air-compressor  is  its  own  dynamometer,  39. 

Air-compressor  diagram,  129. 

Air  a  political  factor,  140. 

Air  always  contains  moisture,  148. 

Air  hoisting,  137. 

Air  for  operating  pumps,  138. 

Air  never  freezes.  147. 

Air  quickly  heated  or  cooled,  95. 

Air  readily  receives  or  imparts  heat,  63. 

Air  the  natural  power-transmitter  for  mines,  175. 

Air  used  without  cooling  or  draining,  150. 

Alternate  resistance  in  single-acting  two-stage  tandem  compressor,  83. 

Applications  of  air-compression  diagram,  100. 

Back  pressure  in  two-stage  compression,  77. 

Bad  luck  of  compressed  air,  138. 

197 


198  INDEX. 

Bad  practices  of  pipe-fitters,  1 14. 

Bad  record  of  air  for  pumping  and  its  causes,  169. 

Beginning  of  economical  compression,  53. 

Best  air-compressor  practice,  31. 

Boiling-point  variable,  n. 

Capacity  of  air  for  water,  149. 

Capacity  of  compressor  as  reduced  by  clearance,  60. 

Catalogues  as  diffusers  of  misinformation,  3 

Caution  as  to  use  of  compression  table,  23. 

Cold  as  possible  air  for  compression,  54. 

Cold-water  fountains,  144. 

Common  working  pressure  for  air,  70. 

Complicated  operation  of  compression,  75. 

Compound  compression,  25. 

Compressed  air  diagram,  explanation  of,  16. 

Compressed  air  gives  less  power  than  equal  volume  of  steam,  98. 

Compressed-air  problem  (the),  27. 

Compressed-air  literature,  3. 

Compressed-air  transmission,  no. 

Compressed  air  versus  electricity,  135. 

Compressed  air  widely  used  without  freezing  up,  148. 

Compressing  cylinder  always  the  first,  72. 

Compression  completed  in  first  cylinder  of  two-stage  compressor.  72. 

Compression  in  a  single  cylinder,  61. 

Compression-line  of  air  and  steam  expansion-line,  49. 

Compressors  for  continuous  service,  134. 

Compressors  in  general  use,  6l. 

Computing  M.  E.  R.,  44. 

Computing  I.  H.  P.,  46. 

Computing  power  cost  of  compression,  90. 

Computing  power  required  for  compression,  23. 

Condition  of  interior  of  pipes,  113. 

Conditions  of  highest  economy  in  compression,  134. 

Considerations  of  economy  inapplicable,  4. 

Constant  readiness  of  air,  137. 

Constant  work  under  best  conditions,  133. 

Continued  transmission  in  winter,  154. 

Cooling  air  for  caisson  work,  156. 

Cooling  drinking-water,  144. 

Cooling  by  injection,  67. 

Cooling  by  water-jacket,  64. 


INDEX.  199 

Cooling  of  air  at  release,  33. 

Corliss  compressor,  133. 

Corliss  feature  for  selling  rather  than  for  operating,  133. 

Cost  of  air-volume  when  produced  by  reheating,  158. 

Cost  of  compression  only  one  part  of  question  of  economy,  90. 

Definitions  and  general  information,  9. 

Devices  for  equalizing  pressure  to  resistance,  130. 

Diagram  from  first  cylinder  does  not  vary  with  ultimate  pressure,  72. 

Diagram  for  one  volume  of  steam  and  air  expanded,  99. 

Diagram  for  drawing  adiabatic  curve,  49. 

Diagram  for  drawing  isothermal  curve,  48. 

Diagram  of  steam  and  air  expanded  to  one  atmosphere,  101. 

Diagram  of  theoretical  air-compression,  27. 

Diagram  of  practical  air-compression,  29. 

Diagram  of  good  comparison,  68. 

Diagram  of  volumes  after  reheating,  164. 

Diagram  of  pressures  after  reheating,  165. 

Diagram  of  volumetric  relations  of  air  and  water,  142. 

Diagram  showing  no  cooling  of  air  in  early  part  of  stroke,  65. 

Diagrams  from  air-brake  pump,  129. 

Diagrams  from  novel  air-compressor,  132. 

Diagrams  in  compressor  catalogues,  127. 

Diagrams  of  two-stage  compression  in  single-acting  cylinders,  71. 

Diagrams  combined  for  double-acting  cylinders,  84. 

Difference  between  diagrams  from  air  and  steam  cylinders,  40. 

Difference  between  theoretical  and  actual  temperatures,  26. 

Difference  between  theory  and  practice,  28. 

Differences  in  free-air  volumes  due  to  temperature,  55. 

Different  effects  of  heat  upon  air  and  water,  141. 

Difficulty  of  learning  the  truth  of  air-compression  practice,  126. 

Distinct  operations  in  air-compression  (two),  24. 

Distributing  pipes,  in. 

Distribution  of  air  in  relation  to  intercooler,  85. 

Drawing  the  adiabatic  curve,  49. 

Drawing  the  isothermal  curve,  48. 

Drinking-fountains  with  warm  water,  144. 

Dry  air  from  injection  compressors,  152. 

Dry-goods  box  and  tumbler,  142. 

Economical  compression,  53. 

Effect  of  heat  on  compressed  air,  14. 

Effect  of  intercooler,  85. 


2OO  INDEX. 

Effect  realized  in  mining-pumps,  36. 

Efficiencies  in  use  of  air,  33. 

Electric  brake,  137. 

Electricity  generally  inapplicable  for  the  work  that  compressed  air 

does,  136. 

Electricity  on  railroads,  139. 
Entrained  water  in  air-meters,  151. 
Erroneous  ideas  as  to  losses  in  transmission,  in. 
Example  of  pumping  by  air,  169. 
Examples  of  use  of  formula  for  flow  of  air,  117—119. 
Explanation  of  compressed-air  diagram,  16. 
Explanation  of  general  compression  table,  20. 
Explanation  of  practical  compression  diagram,  30. 
Expansion  of  air  by  heat  cheaper  than  steam  production,  157. 
Factors  in  transmission  computations,  112. 
Fahrenheit  scale,  n. 

False  position  accepted  by  compressed  air, 
Five  C's  (the),  133. 
Fly-wheels  on  compressors,  130. 
Formula  for  friction  of  air  in  pipes,  115. 
Fountain  cooled  by  compressed  air,  144. 
Four  sources  of  loss  in  air-compression,  93. 
Free  air,  definition,  10. 

Free  air  the  raw  material  of  compression,  54. 
Freezing  up  most  frequent  with  low  pressures,  155. 
Freezing  up  of  air-motors,  34. 
Friction  of  engine,  39. 
Friction  in  straight-line  compressors,  94. 
Gas-engine,  135. 

General  Compressed  Air  Company,  Where  ?  2. 
General  compressed-air  service,  145. 
Getting  the  air  not  only  at  but  into  the  cylinder,  56. 
Giving  a  dog  a  bad  name,  2. 
Governing  the  compressor,  134. 
Graphical  study  of  two-stage  compression,  81. 
Great  changes  of  temperature  with  small  transfers  of  heat,  143. 
Growing  demand  for  compressed  air,  6. 
Heat  not  evenly  distributed  in  cylinder  parts,  66. 
Heat  mostly  abstracted  in  latter  part  of  stroke,  64. 
Heating  and  cooling  of  compressor  parts,  63. 
Heating  ceases  when  compression  ceases,  65. 


INDEX.  2O I 

Heating  effect  of  compression,  15. 

High-pressure  air  and  dry  air,  155. 

High  pressures  and  pressure-reducers,  156. 

How  not  to  do  it,  35. 

Importance  of  the  unimportant,  114. 

Indicator-diagram  must  not  be  taken  too  early,  41. 

Indicator  does  not  tell  weight  of  air  compressed.  57. 

Indicator  on  the  air-compressor  (the),  38. 

Indicator  peculiarly  applied  to  air-compressor,  39. 

Intercooler,  86. 

Isothermal  compression,  15. 

Isothermal  curve,  to  draw.  48. 

Keeping  the  air  cool  means  actual  cooling,  62. 

Large  compressors  for  continuous  service,  134. 

Less  than  nothing  to  do,  82. 

Limits  to  reheating,  161. 

List  of  applications  of  air,  176. 

Little  competition  between  air  and  electricity,  135. 

Little  heat  for  reheating  air,  157. 

Little  power  lost  by  clearance,  60. 

Little  storage  of  air  possible,  135. 

Loss,  the  word  misleading,  36. 

Loss  by  elbows,  114. 

Loss  by  friction  in  two-stage  compression,  80. 

Loss  in  compression  not  necessarily  final,  31. 

Loss  in  transmission,  32. 

Loss  in  transmission  not  due  to  friction,  160. 

Loss  of  pressure  compensated  for  by  increase  of  volume,  32. 

Losses  by  hot  free  air,  57. 

Losses  less  in  air-transmission  than  with  any  other  transmitter,  Tile 

Low  friction  in  Corliss  compressors,  94. 

M.  E.  R.  for  compression  lower  than  for  delivery,  73. 

M.  E.  R.  different  in  single  and  in  two-stage  compression,  74. 

M.  E.  R.  for  compression  only,  24. 

M.  E.  R.  for  whole  stroke,  22. 

M.  E.  R.  single  and  two-stage  compound,  76. 

Measuring  clearance,  44. 

Measuring  total  volume  compressed,  44,  47. 

Mechanical  versus  commercial  economy,  I. 

More  air-pressure  behind  piston  than  in  front,  82. 

Much  air  to  cool  a  little  water,  145. 


2O2  INDEX. 

Novel  arrangement  for  equalizing  pressures,  130. 

Obstructions  in  pipes,  114. 

Oil-engine,  136. 

Operating  pumps,  138 

Operation  of  intercooler,  86. 

Paradox  in  use  of  air,  102. 

Peculiar  position  of  compressed  air,  2. 

Pipe  conveying  air  to  compressor,  55. 

Pohle  air  lift-pump,  143. 

Postal  transmission,  5. 

Power  cost  of  air,  90. 

Power  required  for  hoisting  and  pumping,  167. 

Power  value  of  air,  98. 

Practical  man  has  no  use  for  small  figures,  55. 

Pumping  a  field  for  comparison  with  electricity,  174. 

Pumping  should  show  high  efficiency,  168. 

Rapid  cooling  of  air  in  pipes,  143. 

Ratio  of  cylinders,  71. 

Reading  the  compression-diagram,  38. 

Receiver  near  compressor  does  not  dry  the  air,  151. 

Receiver  needed  after  air  is  cooled,  152. 

Reheating,  34,  157. 

Reheating  generally  impracticable,  163. 

Reheating  in  Paris,  163. 

Reheating  is  doing  work  over  again,  159. 

Relation  of  volume  to  temperature,  n. 

Relative  work  of  low-  and  high-pressure  cylindera,  79. 

Ready-made  pumps,  172. 

Rock-drill  and  air-compressor  (the).  167. 

Saving  by  reheating,  34. 

Simultaneous  heating  and  cooling,  63. 

Single-acting  tandem  two-stage  compressors,  77. 

Single  cylinders  mostly  used,  70. 

Size  of  compression-cylinders,  69. 

Small  compressors  as  missionaries,  8. 

Specific  heat  of  air  and  of  water,  14. 

Starting  business  under  favorab  e  conditions,  62. 

Steam-pressures  guarantee  temperatures,  57. 

Steam-pumps,  169,  171. 

Summary  of  relations,  12. 

Table  of  weights  and  volumes  of  .dry  air,  13. 


INDEX.  203 

Table  of  volumes,  mean  pressures,  etc.,  19. 

Table  of  final  temperatures,  37. 

Table  of  absolute  pressures,  boiling-points,  etc.,  52. 

Table  of  power  required  to  compress  air,  92. 

Table  of  mean  effective  and  terminal  pressures,  103-108. 

Table  of  volumes  of  air  flowing  in  pipes,  109. 

Table  of  relative  volumes  of  air  at  different  pressures,  119. 

Table  of  head  required  to  overcome  friction  in  pipes,  121-125. 

Temperature  of  air  in  cylinder  not  ascertainable,  58. 

Thermal  relations  of  air  and  water,  141. 

Theoretical  compression,  28. 

Transmission  formulas  unsatisfactory,  112. 

Triumph  of  the  steam-engineer  in  electrical  developments,  139. 

Tumbler  and  the  dry-goods  box,  142. 

Two- stage  compression,  31,  70. 

Two-stage  compression  in  single-acting  cylinders,  71. 

Two  things  combine  to  cause  freezing  up,  148. 

Unique  among  power-transmitters,  5. 

Unique  opportunity  of  electricity,  170. 

Unit  of  heat,  14. 

Ultimate  economy  in  reheating,  162. 

Up-to-date  compressor  (the),  126. 

Use  of  air  for  old  steam  pumps,  35. 

Use  of  air  in  railroad  shops,  7. 

Use  of  table  of  power  cost,  93. 

Various  efficiencies  in  use  of  air,  33. 

Various  ratios  of  the  four  losses  in  compression,  97. 

Vindication  of  air-brake  pump,  128. 

Warm  air  in  blowing  cylinders,  59. 

Water  in  compression-cylinders  and  lubrication,  67. 

Water-jacket,  64. 

Water  will  not  wet  what  is  wet,  153. 

Wastefulness  of  air-brake  pump,  4. 

Wet  compressor  furnishes  driest  air,  153. 

Whitewashing  apparatus,  6. 

Without  loss  or  gain  of  heat,  15. 

Worst  air-compressor  in  existence,  128. 


•r  rut 
UNIVERSITY 


SHORT-TITLE     CATALOGUE 

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Brush  and  Penfield's  Manual  of  Determinative  Mineralogy 8vo,  4  oo 

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Fresenius's  Manual  cf  Qualitative  Chemical  Analysis.     (Wells) 8vo,  5  oo 

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4 


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Smith's  Lecture  Notes  on  Chemistry  for  Dental  Students 8vo,  2  50 

Spencer's  Handbook  for  Cane  Sugar  Manufacturers i6mo,  mor.  3  oo 

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Washington's  Manual  of  the  Chemical  Analysis  of  Rocks 8vo,  2  oo 

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CIVIL  ENGINEERING. 

BRIDGES  AND   ROOFS.     HYDRAULICS.     MATERIALS   OF    ENGINEER- 
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Crandall's  Text-book  on  Geodesy  and  Least  Squares 8vo,  3  oo 

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Howe's  Retaining  Walls  for  Earth I2mo,  I  25 

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Sheep,  6  50 

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BRIDGES  AND   ROOFS. 

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7 


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Folwell's  Water-supply  Engineering 8vo,  4  oo 

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Fuertes's  Water  and  Public  Health I2mo,  i  50 

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Hazen's  Clean  Water  and  How  to  Get  It Large  i2tno,  i  50 

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Hazlehurst's  Towers  and  Tanks  for  Water-works 8vo,  2  50 

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Wilson's  Irrigation  Engineering Small  8vo,  4  oo 

Wolff's  Windmill  is  a  Prime  Mover 8vo,  300 

Wood's  Elements  of  Analytical  Mechanics 8vo,  3  oo 

Turbines 8vo,  2  50 

8 


MATERIALS  OF  ENGINEERING. 


Baker's  Roads  and  Pavements 8vo,  5  oo 

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Birkmire's  Architectural  Iron  and  Steel. . 8vo,  3  53 

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Burr's  Elasticity  and  Resistance  of  the  Materials  of  Engineering 8vo,  7  50 

Byrne's  Highway  Construction 8vo,  5  oo 

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Church's  Mechanics  of  Engineering 8vo,  6  oo 

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Fowler's  Ordinary  Foundations 8vo,  3  50 

Graves's  Forest  Mensuration 8vo,  4  oo 

Green's  Principles  of  American  Forestry I2mo,  i  50 

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HoIJy  and  Ladd's  Analysis  of  Mixed  Paints,  Color  Pigments  and  Varnishes 

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Johnson's  (J.  B.)  Materials  of  Construction Large  8vo,  6  oo 

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Kidder's  Architects  and  Builders'  Pocket-book i6mo,  5  oo 

Lanza's  Applied  Mechanics 8vo,  7  50 

Maire's  Modern  Pigments  and  their  Vehicles I2mo,  2  oo 

Martens's  Handbook  on  Testing  Materials.     (Henning)     2  vols 8vo,  7  50 

Maurer's  Technical  Mechanics 8vo,  4  oo 

Merrill's  Stones  for  Building  and  Decoration 8vo,  5  oo 

Merriman's  Mechanics  of  Materials 8vo,  5  oo 

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Metcalfs  Steel.     A  Manual  for  Steel-users I2mo,  a  oo 

Morrison's  Highway  Engineering ^ 8vo,  2  50 

Patton's  Practical  Treatise  on  Foundatio'ns 8vo.  5  oo 

Rice's  Concrete  Block  Manufacture 8vo,  2  oo 

Richardson'a  Modern  Asphalt  Pavements .  .8vo,  3  oo 

Richey's  Handbook  for  Superintendents  of  Construction i6mo,  mor.  4  oo 

*  Ries's  Clays:  Their  Occurrence,  Properties,  and  Uses 8vo,  5  oo 

Sabin's  Industrial  and  Artistic  Technology  of  Paints  and  Varnish .8vo,  3  oo 

«  Schwarz's  Lonsjleaf  Pine  in  Virgin  Forest "mo,  i 

Snow's  Principal  Species  of  Wood 8™.  3 

Spaldins's  Hydraulic  Cement 

Text-book  on  Roads  and  Pavements .......  lamo.  2 

Taylor  and  Thompson's  Treatise  on  Concrete.  Plain  and  Reinforced 8vo,  5  oo 

Thurston's  Materials  of  Engineering.     In  Three  Parts. ...                   8vo,  8  oo 

Parti.     Non-metallic  Materials  of  Engineering  and  Metallurgy 8vo,  2 

Partll.     Iron  and  Steel 8vo.  3  SO 

Part  HI.     A  Treatise  on  Brasses.  Bronzes,  and  Other  Alloys  and  their 

Constituents 8vo'  2  SO 

Tillson's  Street  Pavements  and  Paving  Materials 8vo,  4  o( 

Turneaure  and  Maurer's  Principles  of  Reinforced  Concrete  Construction..  .8vo.  3 

Waterbury's  Cement  Laboratory  Manual «mo,  i  oo 


Wood's  (De  V.)  Treatise  on  the  Resistance  of  Materials,  and  an  Appendix  on 

the  Preservation  of  Timber 8vo,  2  oo 

Wood's  (M.  P.)  Rustless  Coatings:  Corrosion  and  Electrolysis  of  Iron  and 

Steel 8vo,  4  oo 

RAILWAY  ENGINEERING. 

Andrews's  Handbook  for  Street  Railway  Engineers 3x5  inches,  mor.  i   25 

Berg's  Buildings  and  Structures  of  American  Railroads 4to,  5  oo 

Brooks's  Handbook  of  Street  Railroad  Location. i6mo,  mor,  i  50 

Butt's  Civil  Engineer's  Field-book i6mo,  mor.  2  50 

CrandalPs  Railway  and  Other  Earthwork  Tables 8vo,  i  50 

Transition  Curve i6mo,  mor.  i  50 

*  Crockett's  Methods  for  Earthwork  Computations 8vo,  i  50 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book i6mor  rnor.  5  oo 

Dredge's  History  of  the  Pennsylvania  Railroad:    (1879) Paper,  5  oo 

Fisher's  Table  of  Cubic  Sfards Cardboard,  25 

Godwin's  Railroad  Engineers'  Field-book  and  Explorers'  Guide.  .  .  i6mo,  mor.  2  50 
Hudson's  Tables  for  Calculating  the  Cubic  Contents  of  Excavations  and  Em- 
bankments  8vo,  i  oo 

Ives   and  Hilts's   Problems  in  Surveying,  Railroad  Surveying  and  Geodesy 

i6mo,  mor.  i  50 

Molitor  and  Beard's  Manual  for  Resident  Engineers i6mo,  i  oo 

Nagle's  Field  Manual  for  Railroad  Engineers i6mo,  mor.  3  oo 

Philbrick's  Field  Manual  for  Engineers i6mo,  mor.  3  oo 

Raymond's  Railroad  Engineering.     3  volumes. 

VoL      I.  Railroad  Field  Geometry.     (In  Preparation.) 

Vol.    II.  Elements  of  Railroad  Engineering 8vo,  3  50 

Vol.  III.  Railroad  Engineer's  Field  Book.     (In  Preparation.) 

Searles's  Field  'Engineering i6mo,  mor.  3  oo 

Railroad  Spiral i6aio,  mor.  i  50 

Taylor's  Prismoidal  Formulas  and  Earthwork 8vo,  i  50 

*Trautwine's  Field  Practice  of  Laying   Out  Circular  Curves   for  Railroads. 

*  Method  of  Calculating  the  Cubic  Contents  of  Excavations  and  Embank- 

ments by  the  Aid  of  Diagrams 8vo,  2  oo 

Webb's  Economics  of  Railroad  Construction Large  izmo,  2  50 

Railroad  Construction i6mo,  mor.  5  oo 

Wellington's  Economic  Theory  of  the  Location  of  Railways Small  8vo,  5  oo 

DRAWING. 

Barr's  Kinematics  of  Machinery 8vo,  2  50 

*  Bartlett's  Mechanical  Drawing 8vo,  3  oo 

*  "                   "              "            Abridged  Ed 8vo,  i  50 

Coolidge's  Manual  of  Drawing 8vo.  paper,  i  oo 

Coolidge  and  Freeman's  Elements  of  General  Drafting  for  Mechanical  Engi- 
neers  Oblong  410,  2  50 

Durley's  Kinematics  of  Machines 8vo,  4  oo 

E inch's  Introduction  to  Projective  Geometry  and  its  Applications 8vo,  2  50 

Hill's  Text-book  on  Shades  and  Shadows,  and  Perspective 8vo,  2  oo 

Jamison's  Advanced  Mechanical  Drawing 8vo,  2  oo 

Elements  of  Mechanical  Drawing 8vo,  2  50 

Jones's  Machine  Design: 

Part  I.     Kinematics  of  Machinery 8vo,  i  50 

Part  II.    Form,  Strength,  and  Proportions  of  Parts 8vo,  3  oo 

MacCord's  Elements  of  Descriptive  Geometry. 8vo,  3  oc 

Kinematics ;  or.  Practical  Mechanism 8vo,  5  oo 

Mechanical  Drawing 4to,  4  oo 

Velocity  Diagrams , 8vo,  i  50 

10 


McLeod's  Descriptive  Geometry Large  i2mo,  i  50 

*  Mahan's  Descriptive  Geometry  and  Stone-cutting 8vo,  I  50 

Industrial  Drawing.  (Thompson).  . 8vo,  3  50 

Moyer's  Descriptive  Geometry 8vo,  2  oo 

Reed's  Topographical  Drawing  and  Sketching 4to,  5  oo 

Reid's  Course  in  Mechanical  Drawing 8vo,  2 '  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design. 8vo,  3  oo 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Schwamb  and  Merrill's  Elements  of  Mechanism 8vo,  3  oo 

Smith's  (R.  S.)  Manual  of  Topographical  Drawing.  (McMillan) 8vo,  2  50 

Smith  (A.  W.)  and  Marx's  Machine  Design 8vo,  3  oo 

*  Titsworth's  Elements  of  Mechanical  Drawing Oblong  8vo,  i  25 

Warren's  Drafting  Instruments  and  Operations 121110,  i  25 

Elements  of  Descriptive  Geometry,  Shadows,  and  Perspective 8vo,  3  50 

Elements  of  Machine  Construction  and  Drawing 8vo,  7  50 

Elements  of  Plane  and  Solid  Free-hand  Geometrical  Drawing izmo,  i  oo 

General  Problems  of  Shades  and  Shadows 8vo,  3  oo 

Manual  of  Elementary  Problems  in  the  Linear  Perspective  of  Form  and 

Shadow i2mo,  i  oo 

Manual  of  Elementary  Projection  Drawing ismo,  i  50 

Plane  Problems  in  Elementary  Geometry 12010,  i  25 

Problems,  Theorems,  and  Examples  in  Descriptive  Geometry 8vo,  a  50 

Weisbach's    Kinematics    and    Power    of    Transmission.        (Hermann    and 

Klein) 8vo,  5  oo 

Wilson's  (H.  M.)  Topographic  Surveying 8vo,  3  50 

Wilson's  (V.  T.)  Free-hand  Lettering 8vo,  i  oo 

Free-hand  Perspective 8vo,  2  50 

Woolf' s  Elementary  Course  in  Descriptive  Geometry Large  8vo,  3  oo 

ELECTRICITY  AND  PHYSICS. 

*  Abegg's  Theory  of  Electrolytic  Dissociation,     (von  Ende) I2mo,  i   25 

Andrews's  Hand-Book  for  Street  Railway  Engineering    ...  .3X5  inches,  mor.  i  25 

Anthony  and  Brackett's  Text-book  of  Physics.     (Magie). Large  I2mo,  3  oo 

Anthony's  Theory  of  Electrical  Measurements.     (Ball) I2mo,  i  oo 

Benjamin's  History  of  Electricity 8vo,  3  oo 

Voltaic  Cell. 8vo,  3  oo 

Betts's  Lead  Refining  and  Electrolysis 8vo,  4  oo 

Classen's  Quantitative  Chemical  Analysis  by  Electrolysis.     (Boltwood).  .8vo,  3  oo 

*  Collins's  Manual  of  Wireless  Telegraphy I2mo,  i  50 

Mor.  2  oo 

Crehore  and  Squier's  Polarizing  Photo-chronograph 8vo,  3  oo 

*  Danneel's  Electrochemistry.     (Merriam) I2mo,  i  25 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book  ....  i6mo,  mor.  5  oo 
Dolezalek's  Theory  of  the  Lead  Accumulator  (Storage  Battery),      (von  Ende) 

I2mo,  2  50 

Duhem's  Thermodynamics  and  Chemistry.     (Burgess) 8vo,  4  oo 

Flather's  Dynamometers,  and  the  Measurement  of  Power I2mo,  3  oo 

Gilbert's  De  Magnete.     (Mottelay) 8vo,  2  50 

*  Hanchett's  Alternating  Currents "mo,  i  oo 

Bering's  Readv  Reference  Tables  (Conversion  Factors) i6mo,  mor.  2  50 

*  Hob  art  and  Ellis's  High-speed  Dynamo  Electric  Machinery 8vo,  6  oo 

Holman's  Precision  of  Measurements 8vo,  2  oo 

Telescopic   Mirror-scale  Method,  Adjustments,  and  Tests Large  8vc,  75 

*  Karapetoff's  Experimental  Electrical  Engineering 8vo,  6  oo 

Kinzbrunner's  Testing  of  Continuous-current  Machines 8vo,  a  oo 

Landauer's  Spectrum  Analysis.     (Tingle) 8vo,  3  oo 

Le  Chatelier's  Hi?h-temperature  Measurements.  (Boudouard — Burgess).. I2mo.  3  oo 

Lob's  Electrochemistry  of  Organic  Compounds.     (Lorenz) 8vo,  3  oo 

*  Lyndon's  Development  and  Electrical  Distribution  of  Water  Fower  . . .  .8vo,  3  oo 

11 


*  Lyons's  Treatise  on  Electromagnetic  Phenomena.   Vols.  I.  and  II,  8vo,  each,  6  oo 

*  Michie's  Elements  of  Wave  Motion  Relating  to  Sound  and  Light 8vo,  4  oo 

Morgan's  Outline  of  the  Theory  of  Solution  and  its  Results I2mo,  i  oo 

*  Physical  Chemistry  for  Electrical  Engineers izrno,  i  50 

Niaudet's  Elementary  Treatise  on  Electric  Batteries.     (Fishback).  . .  .  I2mo,  2   50 

*  Norris's  Introduction  to  the  Study   of  Electrical  Engineering 8vo,  2  50 

*  Parshall  and  Hobart's  Electric  Machine  Design 4to,  half  mor.  12  50 

Reagan's  Locomotives:    Simple,  Compound,  and  Electric.      New  Edition. 

Large  I2mo,  3  50 

*  Rosenberg's  Electrical  Engineering.     (Haldane  Gee— Kinzbrunner).  . .  .8vo,  2  co 

Ryan,  Norris,  and  Hoxie's  Electrical  Machinery.     Vol.  1 8vo,  2  50 

Schapper's  Laboratory  Guide  for  Students  in  Physical  Chemistry i2mo,  i  oo 

*  Tillman's  Elementary  Lessons  in  Heat 8vo,  i  50 

Tory  and  Pitcher's  Manual  of  Laboratory  Physics Large  I2mo,  2  cx> 

Ulke's  Modern  Electrolytic  Copper  Refining 8vo.  3  oo 

LAW. 
Brennan's  Handbook :   A  Compendium    of    Useful   Legal  Information   for 

Business  Men i6mo,  mor.  5 .  oo 

*  Davis's  Elements  or  Law 8vo,  2  50 

*  Treatise  on  the  Military  Law  of  United  States 8vo,  7  oo 

*  Sheep,  7  50 

*  Dudley's  Military  Law  and  the  Procedure  of  Courts-martial Large  121110,  2  50 

Manual  for  Courts-martial ibmo,  mor.  i  50 

Wait's  Engineering  and  Architectural  Jurisprudence 8vo,  6  oo 

Sheep,  6  50 

Law  of  Contracts 8vo,  3  oo 

Law  of  Operations  Preliminary  to  Construction  in  Engineering  and  Archi- 
tecture  8vo  5  oo 

Sheep,  5  50 
MATHEMATICS. 

Baker's  Elliptic  Functions 8vo,  i  50 

Briggs's  Elements  of  Plane  Analytic  Geometry.    (B6cher) I2mo,  i  oo 

*Buchanan's  Plane  and  Spherical  Trigonometry 8vo,  i  oo 

Byerley's  Harmonic  Functions 8vo,  i  oo 

Chandler's  Elements  of  the  Infinitesimal  Calculus I2mo,  2  oo 

Coffin's  Vector  Analysis.         (In  Press.) 

Compton's  Manual  of  Logarithmic  Computations I2mo,  i   50 

*  Dickson's  College  Algebra Large  I2ino,  i   50 

*  Introduction  to  the  Theory  of  Algebraic  Equations Large  I2mo,  i   25 

Emch's  Introduction  to  Projective  Geometry  and  its  Applications 8vo,  2  50 

Fiske's  Functions  of  a  Complex  Variable .  .  .  • 8vo,  i  oo 

Halsted's  Elementary  Synthetic  Geometry 8vo,  i   50 

Elements  of  Geometry 8vo,  i   75 

*  Rational  Geometry izmo,  i   50 

Hyde's  Grassmann's  Space  Analysis 8vo,  i  oo 

*  Jonnson's  (J   B,)  Three-place  Logarithmic  Tables:  Vest-pocket  size,  paper,  15 

100  copies,  5  oo 

*  Mounted  on  heavy  cardboard,  8  X 10  inches,  25 

10  copies,  2  oo 
Johnson's  (W.  W.)  Abridged  Editions  of  Differential  and  Integral  Calculus 

Large  I2mo,  i  vol.  2  50 

Curve  .'racing  in  Cartesian  Co-ordinates I2mo,  i  oo 

Differential  Equations 8vo.  T  oo 

Elementary  Treatise  on  Differential  Calculus Large  12010,  i  50 

Elementary  Treatise  on  the  Integral  Calculus ; .  .Large  I2mo.  t  50 

*  Theoretical  Mechanics I2mo,  3  oo 

Theory  of  Errors  and  the  Method  of  Least  Squares I2mo,  i  50 

Treatise  on  Differential  Calculus Latge  121110,  3  oo 

12 


Johnson's  Treatise  on  the  Integral  Calculus. Large  i2mo,    3  OD 

Treatise  on  Ordinary  and  Partial  Differential  Equations. .  Large  12010,    3  50 
Karapetofi's   Engineering  Applications  of  Higher  Mathematics.      (In   Pre- 
paration.) 
Laplace's  Philosophical  Essay  on  Probabilities.     (Truscott  and  Emory). .I2mo,     2  oo 

*  Ludlow  and  Bass's  Elements  of  Trigonometry  and  Logarithmic  and  Other 

Tables 8vo,     j  oo 

Trigonometry  and  Tables  published  separately Each,     2  oo 

*  Ludlow's  Logarithmic  and  Trigonometric  Tables 8vo,     i  oo 

Macfarlane's  Vector  Analysis  and  Quaternions 8vo,     I  oo 

McMahon's  Hyperbolic  Functions 8vo,     i  oo 

Manning's  Irrational  Numbers  and  their  Representation  by  Sequences  and 

Series I2mo,     i   25 

Mathematical  Monographs.     Edited  by  Mansfield  Merriman  and  Robert 

S.  Woodward „ Octavo,  each     i  oo 

No.  i.  History  of  Modern  Mathematics,  by  David  Eugene  Smith. 
No.  2.  Synthetic  Projective  Geometry,  by  George  Bruce  Halsted. 
No.  3.  Determinants,  by  Laenas  Gifford  Weld.  No.  4.  Hyper- 
bolic Functions,  by  James  McMahon.  Ko.  s.  Harmonic  Func- 
tions, by  William  E.  Byerly.  No.  6.  Grassmann's  Space  Analysis, 
by  Edward  W.  Hyde.  No.  7.  Probability  and  Theory  of  Errors, 
by  Robert  S.  Woodward.  No.  8.  Vector  Analysis  and  Quaternions, 
by  Alexander  Macfarlane.  No.  9.  Differential  Equations,  by 
William  Woolsey  Johnson.  No.  10.  The  Solution  of  Equations, 
by  Mansfield  Merriman.  No.  n.  Functions  of  a  Complex  Variable, 
by  Thomas  S.  Fiske. 

Maurer's  Technical  Mechanics 8vo,    4  oo 

Merriman's  Method  of  Least  Squares 8vo,    2  oo 

Solution  of  Equations 8vo,     i   oo 

Rice  and  Johnson's  Differential  and  Integral  Calculus.     2  vols.  in  one. 

Large  I2mo,     i  50 

Elementary  Treatise  on  the  Differential  Calculus Large  I2mo,     3  oo 

Smith's  History  of  Modern  Mathematics 8vo,     i  oo 

*  Veblen  and  Lennes's  Introduction  to  the  Real  Infinitesimal  Analysis  of  One 

Variable 8vo,     2  oo 

*  Waterbury's  Vest  Pocket  Hand-Book  of  Mathematics  for  Enginef  rs. 

2jX 5 J  inches,  mor.     i  oo 

Weld's  Determinations 8vo,     i  co 

Wood's  Elements  of  Co-ordinate  Geometry 8vo,    2  oo 

Woodward's  Probability  aid  Theory  of  Errors 8vo,    i  oo 

MECHANICAL  ENGINEERING. 
MATERIALS  OF   ENGINEERING,  STEAM-ENGINES  AND  BOILERS. 

Bacon's  Forge  Practice .  12010,  i  50 

Baldwin's  Steam  Heating  for  Buildings I2mo,  2  50 

Batr's  Kinematics  of  Machinery 8vo,  2  50 

*  Bartlett's  Mechanical  Drawing 8vo,  3  oo 

*  "  "  "        Abridged  Ed 8vo,    150 

Benjamin's  Wrinkles  and  Recipes 12010,    2  oo 

*  Burr's  Ancient  and  Modern  Engineering  and  the  Isthmian  Canal 8vo,    3  50 

Carpenter's  Experimental  Engineering 8vo,    6  oo 

Heating  and  Ventilating  Buildings 8vo,  4  oo 

Clerk's  Gas  and  Oil  Engine Large  12010,  4  oo 

Compton's  First  Lessons  in  Metal  Working 12010,  i  50 

Compton  and  De  Groodt's  Speed  Lathe I2mo,  i  50 

Coolidge's  Manual  of  Drawing 8vo,  paper,  i  oo 

Coolidge  and  Freeman's  Elements  of  General  Drafting  for  Mechanical  En- 
gineers  Oblong  4to,  2  50 

13 


Cromwell's  Treatise  on  Belts  and  Pulleys nmo,  i  50 

Treatise  on  Toothed  Gearing 12010,  i  50 

Durley's  Kinematics  of  Machines 8vo,  4  oo 

Flather's  Dynamometers  and  the  Measurement  of  Power, I2mo,  3  oo 

Rope  Driving I2mo,  2  oo 

Gill's  Gas  and  Fuel  Analysis  for  Engineers i2mo,  i  25 

Goss  s  Locomotive  Sparks 8vo,  2  oo 

Greene's  Pumping  Machinery.     (In  Preparation.) 

Bering's  Ready  Reference  Tables  (Conversion  Factors) i6mo,  mor.  2  50 

*  Hobart  and  Ellis's  High  Speed  Dynamo  Electric  Machinery 8vo,  6  oo 

Button's  Gas  Engine 8vo,  5  oo- 

Jamison's  Advanced  Mechanical  Drawing 8vo,  2  oo 

Elements  of  Mechanical  Drawing 8vo,  2  50 

Jones's  Gas  Engine.      (In  Press.) 
Machine  Design: 

Part  I.     Kinematics  of  Machinery 8vo,  i  50 

Part  II.     Form,  Strength,  and  Proportions  of  Parts 8vo,  3  oo 

Kent's  Mechanical  Engineers'  Pocket-book i6mo,  mor.  5  oo 

Kerr's  Power  and  Power  Transmission 8vo,  2  oo 

Leonard's  Machine  Shop  Tools  and  Methods 8vo,  4  oo 

*  Lorenz's  Modern  Refrigerating  Machinery.    (Pope,  Haven,  and  Dean) . .  .  8vo,  4  oo 
MacCord's  Kinematics;   or,  Practical  Mechanism 8vo,  5  oo 

Mechanical  Drawing 4to,  4  oo 

Velocity  Diagrams 8vo,  i'  50 

MacFarland's  Standard  Reduction  Factors  for  Gases 8vo,  i  50 

Mahan's  Industrial  Drawing.      (Thompson) 8vo,  3  50 

Oberg's   Screw   Thread  Systems,    Taps,    Dies,  Cutters,  and   Reamers.       (In 
Press.) 

*  Parshall  and  Hobart's  Electric  Machine  Design Small  4to,  half  leather,  12  50 

Peele's  Compressed  Air  Plant  for  Mines 8vo,  3  oo 

Poole's  Calorific  Power  of  Fuels 8vo,  3  oo 

*  Porter's  Engineering  Reminiscences,  1855  to  1882 8vo,  3  oo 

Reid's  Course  in  Mechanical  Drawing 8vo,  2  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design. 8vo,  3  oo 

Richard's  Compressed  Air I2mo,  i  50 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Schwamb  and  Merrill's  Elements  of  Mechanism 8vo,  3  oo 

Smith's  (O.)  Press- working  of  Metals ., 8vo,  3  oo 

Smith  (A.  W.)  and  Marx's  Machine  Design 8vo,  3  oo 

Sorel '  s  Carbureting  and  Combustion  in  Alcohol  Engines .    (Woodward  and  Preston) . 

Large  12 mo,  3  oo 
Thurston's  Animal  as  a  Machine  and  Prime  Motor,  and  the  Laws  of  Energetics. 

I2H10,  I  00 

Treatise  on  Friction  and  Lost  Work  in  Machinery  and  Mill  Work...  8vo,  3  oo 

Tillson's  Complete  Automobile  Instructor i6mo,  i  50 

mor.  a  oo 

Titsworth's  Elements  of  Mechanical  Drawing .' Oblong  8vo.  i  25 

Warren's  Elements  of  Machine  Construction  and  Drawing 8vo,  7  SO 

*  Waterbury's  Vest  Pocket  Hand  Book  of  Mathematics  for  Engineers. 

zjX  5 J  inches,  mor.  i  oo 
Weisbach's    Kinematics    and    the    Power    of    Transmission.     (Herrmann — 

Klein) 8vo,  5  oo 

Machinery  of  Transmission  and  Governors.     (Herrmann — Klein)..  .8vo,  5  oo 

Wood's  Turbines 8vo,  2  50 

MATERIALS   OF   ENGINEERING 

*  Bovey's  Strength  of  Materials  and  Theory  of  Structures 8vo,  7  50 

Burr's  Elasticity  and  Resistance  of  the  Materials  of  Engineering 8vo,  7  50 

Church's  Mechanics  of  Engineering 8vo,  6  oo 

*  Greene's  Structural  Mechanics 8vo»  2  50 

14 


Holley  and  Ladd's  Analysis  of  Mixed  Paints,  Color  Pigments,  and  Varnishes. 

Large  i2mo,  2  50 

Johnson's  Materials  of  Construction 8vo,  6  oo 

Keep's  Cast  Iron 8vo,  2  50 

Lanza's  Applied  Mechanics 8vo,  7  50 

Maire's  Modern  Pigments  and  their  Vehicles izmo,  2  oo 

Martens's  Handbook  on  Testing  Materials.     (Henning) 8vo,  7  50 

Maurer's  Technical  Mechanics 8vo,  4  oo 

Merriman's  Mechanics  of  Materials 8vo,  5  oo 

*  Strength  of  Materials I2mo,  i  oo 

Metcaif's  Steel.     A  Manual  for  Steel-users izmo,  2  oo 

Sabin's  Industrial  and  Artistic  Technology  of  Paints  and  Varnish 8vo,  3  oo 

Smith's  Materials  of  Machines 12:110,  i  oo 

Thurston's  Materials  of  Engineering 3  vols.,  8vo,  8  oo 

Parti.        Non-metallic  Materials  of  Engineering  and  Metallurgy ..  .8vo,  2  oo 

Part  II.       Iron  and  Steel 8vo,  3  50 

Part  III.     A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and   their 

Constituents 8vo,  2  50 

Wood's  (De  V.)  Elements  of  Analytical  Mechanics 8vo.  3  oo 

Treatise  on    the    Resistance    of    Materials  and    an  Appendix  on  the 

Preservation  of  Timber 8vo,  2  oo 

Wood's  (M.  P.)  Rustless  Coatings:    Corrosion  and  Electrolysis  of  Iron  and 

Steel 8vo,  4  oo 

STEAM-ENGINES  AND  BOILERS. 

Berry's  Temperature-entropy  Diagram 121110,  i  25 

Carnot's  Reflections  on  the  Motive  Power  of  Heat.     (Thurston) I2mo,  i  50 

Chase's  Art  of  Pattern  Making izmo,  2  50 

Creighton's  Steam-engine  an  J  olher  Heat-motors      8vo,  500 

Dawson's  "  Engineering"  and  Electric  Traction  Pocket-book i6mo,  mor.  3  oo 

Ford's  Boiler  Making  for  Boiler  Makers i8mo,  i  oo 

*  Gebhardt's  Steam  Power  Plant  Engineering 8vo,  6  oo 

Goss's  Locomotive  Performance 8vo,  5  oo 

Hemenway's  Indicator  Practice  and  Steam-engine  Economy I2mo,  2  oo 

Button's  Heat  and  Heat-engines 8vo,  5  oo 

Mechanical  Engineering  of  Power  Plants 8vo,  5  oo 

Kent's  Steam  boiler  Economy 8vo,  4  oo 

Kneass's  Practice  and  Theory  of  the  Injector 8vo,  i  50 

MacCord's  Slide-valves 8vo,  2  oo 

Meyer's  Modern  Locomotive  Construction 4to,  10  oo 

Meyer's  Steam  Turbines.      (In  Press.) 

Peabody's  Manual  of  the  Steam-engine  Indicator I2mo.  i  50 

Tables  of  the  Properties  of  Saturated  Steam  and  Other  Vapors 8vo,  i  oo 

Thermodynamics  of  the  Steam-engine  and  Other  Heat-engines 8vo,  5  oo 

Valve-gears  for  Steam-engines .8vo,  2  50 

Peabody  and  Miller's  Steam-boilers 8vo,  4  oo 

Fray's  Twenty  Years  with  the  Indicator Large  8vo,  2  50 

Pupin's  Thermodynamics  of  Reversible  Cycles  in  Gases  and  Saturated  Vapors. 

(Osterberg) I2mo,  i  25 

Reagan's  Locomotives:    Simple,  Compound,  and  Electric.     New  Edition. 

Large  i2mo.  3  <o 

Sinclair's  Locomotive  Engine  Running  and  Management 12 mo,  2  oo 

Smart's  Handbook  of  Engineering  Laboratory  Practice I2mo,  2  50 

Snow's  Steam-boiler  Practice 8vo,  3  oo 

Spangler's  Notes  on  Thermodynamics I2mo,  i  oo 

Valve-gears 8vo,  2  50 

Spangler,  Greene,  and  Marshall's  Elements  of  Steam-engineering 8vo,  3  oo 

Thomas's  Steam-turbines 8vo,  4  oo 

15 


Thurston's  Handbook  of  Engine  and  Boiler  Trials,  and  the  Use  of  the  Indi- 
cator and  the  Prony  Brake. Svo,  5  oo 

Handy  Tables 8vo,  i  50 

Manual  of  Steam-boilers,  their     esigns,  Construction,  and  Operation.. Svo,  5  oo 

Thurston's   Manual  of  the  Steam-engine 2  vols.,  Svo,  10  oo 

Part  I.     History,  Structure,  and  Theory Sv,  6  oo 

Part  II.     Design,  Construction,  and  Operation Svo,  6  oo 

Steam-boiler  Explosions  in  Theory  and  in  PracJce I2mo,    i   50 

Wehrenfenning's  Analysis  and  Softening  of  Boiler  Feed-water  (Patterson)   Svo,  4  oo 

Weisbach's  Heat,  Steam,  and  Steam-engines.     (Du  Bois) Svo,  5  oo 

Whitham's  Steam-engine  Design Svo,  5  oo 

Wood's  Thermodynamics,  Heat  Motors,  and  Refrigerating  Machines.  .  .Svo,  4  oo 

MECHANICS  PURE  AND   APPLIED. 

Church's  Mechanics  of  Engineering Svo,  6  oo 

Notes  and  Examples  in  Mechanics Svo,  2  oo 

Dana's  Text-book  of  Elementary  Mechanics  for  Colleges  and  Schools.  .I2mo,  i  50 
Du  Bois's  Elementary  Principles  of  Mechanics: 

Vol.      I.     Kinematics Svo,  3  50 

Vol.    II.     Statics Svo,  4  oo 

Mechanics  of  Engineering.     Vol.    I Small  4to,  7  50 

Vol.  II. Small  4to,  10  oo 

*  Greene's  Structural  Mechanics Svo,  2  50 

James's  Kinematics  of  a  Point  and  the  Rational  Mechanics  of  a  Particle. 

Large  i2mo,  2  oo 

*  Johnson's  (W.  W.)  Theoretical  Mechanics I2mo,  3  oo 

Lanza's  Applied  Mechanics Svo,  7  50 

*  Martin's  Text  Book  on  Mechanics,  Vol.  I,  Statics lamo,  i  25 

Vol.  2,  Kinematics  and  Kinetics  .  .i2mo,  1  50 

Maurer's  Technical  Mechanics Svo,  4  oo 

*  Merriman's  Elements  of  Mechanics I2mo,  i  oo 

Mechanics  of  Materials Svo,  5  oo 

*  Michie's  Elements  of  Analytical  Mechanics Svo,  4  oo 

Robinson's  Principles  of  Mechanism Svo,  3  oo 

Sanborn's  Mechanics  Problems Large  I2mo,  i  50 

Schwamb  and  Merrill's  Elements  of  Mechanism Svo,  3  oo 

Wood's  Elements  of  Analytical  Mechanics Svo,  3  oo 

Principles  of  Elementary  Mechanics I2mo,  i  25 

ME11CAL. 

*  Abderhalden's  Physiological  Chemistry  in  Thirty  Lectures.    (Hall  and  Defren) 

Svo,  5  oo 

von  Behring's  Suppression  of  Tuberculosis.     (Bolduan) I2mo,  i  oo 

*  Bolduan's  Immune  Sera , 1 2mo,  i  50 

Bordet's  Contribution  to  Immunity.     (Gay).     (In  Preparation.) 

Davenport's  Statistical  Methods  with  Special  Reference  to  Biological  Varia- 
tions  1 6mo ,  mor.  i  50 

Ehrlich's  Collected  Studies  on  Immunity.     (Bolduan) Svo,  6  oo 

*  Fischer's  Physiology  of  Alimentation Large  T2mo,  cloth,  2  oo 

de  Fursac's  Manual  of  Psychiatry.     (Rosanoff  and  Collins) Large  12010,  250 

Hammarsten's  Text-book  on  Physiological  Chemistry.     (Mandel) Svo,  4  oo  . 

Jackson's  Directions  for  Laboratory  Work  in  Physiological  Chemistry    .  .Svo,  i  25 

Lassar-Cohn's  Practical  Urinary  Analysis.     (Lorenz) I2mo,  i  oo 

Mandel's  Hand  Book  for  the  Biochemical  Laboratory I2mo,  i  50 

*  Pauli's  Physical  Chemistry  in  the  Service  of  Medicine.     (Fischer) I2mo,  r  25 

<"  Pozzi-Escot's  Toxins  and  Venoms  and  their  Antibodies.     (Cohn) I2mo,  i  oo 

Rostoski's  Serum  Diagnosis.     (Bolduan) I2mo,  i  oo 

Ruddiman's  Incompatibilities  in  Prescriptions Svo.  2  oo 

Whys  in  Pharmacy "mo,  i  oo 

16 


Salkowsk''s  Physiological  and  Pathological  Chemistry.     (Orndorff) 8vo,  2  S<* 

*  Satterlee's  Outlines  of  Human  Embryology 12010,  i  45 

Smith's  Lecture  Notes  on  Chemistry  for  Dental  Students 8vo,  2  50 

Steel's  Treatise  on  the  Diseases  of  the  Dog 8vo,  3  50 

*  Whippie's  Typhoid  Fever Large  izmo,  3  oo 

Woodhull's  Notes  on  Military  Hygiene i6mo,  r  50 

*  Personal  Hysrlene izmo,  i  oo 

Worcester  and  Atkinson's  Small  Hospitals  Establishment  and  Maintenance, 

and  S  ggestions  for  Hospital  Architecture,  with  Plans  for  a  Small 

Hospital .• i2mo,  i  25 

METALLURGY.      • 

Betts's  Lead  Refining  by  Electrolysis 8vo,  4  oo 

Holland's  Encyclopedia  of  Founding  and  Dictionary  of  Foundry  Terms  Used 

in  the  Practice  of  Moulding 12010,  3  oo 

Iron  Founder 12010,  2  50 

"            "        Supplement I2mo,  2  50 

Douglas's  Untechnical  Addresses  on  Technical  Subjects I2tno,  i  oo 

Goesel's  Minerals  and  Metals:  A  Reference  Book i6mo,  mor.  3  oo 

*  Iles's  Lead-smelting I2mo,  2  50 

Keep's  Cast  Iron 8vo,  2  50 

LeChatelier's  High-temperature  Measurements.   (Boudouard — Burgess)  I2mo,  3  oo 

Metcalf's  Steel.     A  Manual  for  Steel-users I2mo,  2  oo 

Miller's  Cyanide  Process I2mo,  i  oo 

Minet's  Production  of  Aluminium  and  its  Industrial  Use.    (Waldo)  .  .  .i2mo,  2  50 

Robine  and  Lenglen's  Cyanide  Industry.     (Le  Clerc) 8vo,  4  oo 

Ruer's  Elements  of  Metallography.     (Mathewsoa)     (In  Press.) 

Smith's  Materials  of  Machines I2mo,  r  oo 

Tate  ar>.d  Stone's  Foundry  Practice.     (In  Press.) 

Thurston's  Materials  of  Engineering.     In  Three  Parts 8vo,  8  oo 

Part  I.         Non-metallic  Materials  of  Engineering  and  Metallurgy  .  .  .  8vo,  2  oo 

Part  II.       Iron  and  Steel 8vo,  3  50 

Part  III.     A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and  their 

Constituents 8vo,  2  50 

Ulke's  Modern  Electrolytic  Copper  Refining 8vo,  3  oo 

West's  American  Foundry  Practice I2mo,  2  50 

Moulder's  Text  Book    I2mo,  2  50 

Wilson's  Chlorination  Process i2mo,  i  50 

Cyanide  Processes I2mo,  i  50 

MINERALOGY. 

Barringer's  Description  of  Minerals  of  Commercial  Value Oblong,  mor.  2  50 

Boyd's  Resources  of  Southwest  Virginia 8vo,  3  oo 

Boyd's  Map  of  Southwest  Virginia. Pocket-book  form.  2  oo 

*  Browning's  Introduction  to  the  Rarer  Elements 8vo,  i   50 

Brush's  Manual  of  Determinative  Mineralogy.     (Penfield) *. 8vo,  4  oo 

Butler's  Pocket  Hand-Book  of  Minerals i6mo,  mor.  3  oo 

Chester's  Catalogue  of  Minerals 8vo,  paper,  i  oo 

Cloth,  i   25 

*Crane's  Gold  and  Silver 8vo,  5  oo 

Dana's  First  Appendix  to  Dana's  New  "  System  of  Mineralogy. ." .   Large  8vo,  i  oo 

Manual  of  Mineralogy  and  Petrography i?.mo  2  oo 

Minerals  and  How  to  Study  Them ...  izmo.  i  50 

System  of  Mineralogy Large  8vo,  half  leather,  12  50 

Textbook  of  Mineralogy 8vo,  4  oo 

Douglas's  Untechnical  Addresses  on  Technical  Subjects I2mo,  i  oo 

Eakle's  Mineral  Tables •  •   8vo,  i  25 

Stone  and  Clay  Products  Used  in  Engineering.     (In  Preparation.) 
17 


Egleston's  Catalogue  of  Minerals  and  Synonyms 8vo,  2  50 

Goesel's  Minerals  and  Metals:     A  Reference  Book i6mo,  mor.  3  oo 

broth's  Introduction  to  Chemical  Crystallography  (Marshall) 12 mo,  i  25 

*  Iddmes's  Rock  Minerals    .                                 ....   Svo,  5  oo 

Johannsen's  Determination  of  Rock-forming  Minerals  in  Thin  Sections 8vo,  4  oo 

*  Martin's  Laboratory  Guide  to  Qualitative  Analysis  with  the  Blowpipe.  I2mo,  60 
Merrill's  Non-metallic  Minerals:   Their  Occurrence  and  Uses 8vo,  4  oo 

Stones  for  Building  and  Decoration 8vo,  5  oo 

*  Penfield's  Notes  on  Determinative  Mineralogy  and  Record  of  Mineral  Tests. 

8vo,  paper,  50 
Tables    of    Minerals,    Including   the  Use  of  Minerals  and  Statistics  of 

Domestic  Production 8vo,  i  oo 

*  Pirsson's  Rock?  and  Rock  Minerals 1 2mo,  2  50 

*  Richards's  Synopsis  of  Mineral  Characters i2mo,  mor.  i  25 

*  Ries's  Clays:  Their  Occurrence,  Properties,  and  Uses..  .  .    8vo,  5  oo 

*  Tiilman's  Text-book  of  Important  Minerals  and  Rocks 8vo,  2  oo 

MINING. 

*  Beard's  Mine  Gases  and  Explosions Large  i2mo,  3  oo 

Boyd's  Map  of  Southwest  Viiginia Pocket-book  lorm  2  oo 

Resources  of  Southwest  Virginia 8vo,  3  oo 

*  Crane's  Gold  and  Silver    8vo,  5  oo 

Douglas's  Untechnical  Addresses  on  Technical  Subjects I2mo:  i  oo 

Eissler's  Modern  Hi^h  Explosives 8vo,  4   --o 

Goesel's  Minerals  and  Metals:     A  Reference  Book i6mo,  mor.  3  oo 

I   Iseng's  Manual  ot  Mining 8vo,  5  oo 

*  Iles's  Lead-smeltin?. . I2mo,  2  50 

Miller's  Cyanide  Process I2mo,  i  oo 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores Svo,  2  oo 

Peele's  Compressed  Air  Plant  for  Mines Svo,  3  oo 

Riemer's  Shaft  Sinking  Under  Difficult  Conditions.     (Corning  and  Peele) ...  Svo,  3  oo 

Robine  and  Lenglen's  Cyanide  Industry.     (Le  Clerc) Svo,  4  oo 

*  Weaver's  MTtary  Explosives Svo,  3  oo 

Wilson's  Chlorination  Process ismo,  i  50 

Cyanide  Processes I2mo,  i  50 

Hydraulic  and  Placer  Mining.     2rf  edition,  rewritten I2mo,  2  50 

Treatise  on  Practical  and  Theoretical  Mine  Ventilation I2mo,  i  25 

SANITARY  SCIENCE. 

Association  of  State  and  National  Food  and  Dairy  Departments,  Hartford  Meeting, 

1906 Svo,  3  oo 

Jamestown  Meeting,  1907 Svo,  3  oo 

*  Bashore's  Outlines  of  Practical  Sanitation I2mo,  i  25 

Sanitation  of  a  Country  House 1 2mo,  i  oo 

Sanitation  of  Recreation  Camps  and  Parks I2mo,  i  oo 

Folwell's  Sewerage.  (Designing,  Construction,  and  Maintenance) Svo,  3  oo 

Water-supply  Engineering Svo,  4  oo 

Fowler's  Sewage  Works  Analyses I2mo,  2  oo 

Fuertes's  Water-filtration  Works I2mo,  2  50 

Water  and  Public  Health I2mo,  i  50 

Gerhard's  Guide  to  Sanitary  House-inspection i6mo,  i  oo 

*  Modern  Baths  and  Bath  Houses   Svo,  3  oo 

Sanitation  of  Public  Buildings izmo,  i   50 

Hazen's  Ctein  Water  and  How  to  Get  It » Large  i2mo,  i  50 

Filtration  of  Public  Water-supplies Svo,  3  oo 

Kinnicut,  Winslow  and  Pratt's  Purification  of  Sewage.     (In  Press.) 

Leach's    Inso-ction   and    Analysis  of  Food  with  Special  Reference   to  State 

Control 8™.  7  oo 

18 


Mason's  Examination  of  Water.     (Chemical  and  Bacteriological) 12010, 

Water-supply.  (Considered  Principally  from  a  Sanitary  Standpoint; .  .8vo, 

*  Merriman's  Elements  of  Sanitary  Engineering 8vo, 

Ogden's  Sewer  Design I2mo, 

Parsons's  Disposal  of  Municipal  Refuse 8vo, 

Prescott  and  Winslow's  Elements  of  Water  Bacteriology,  with  Special  Refer- 
ence to  Sanitary  Water  Analysis I2mo, 

*  Price's  Handbook  on  Sanitation I2mo, 

Richards's  Cost  of  Cleanness.     A  Twentieth  Century  Problem i2mo, 

Cost  of  Food.     A  Study  in  Dietaries I2mo, 

Cost  of  Living  as  Modified  by  Sanitary  Science i2mo, 

Cost  of  Shelter.    A  Study  in  Economics I2mo, 

*  Richards  and  Williams's  Dietary  Computer 8vo, 

Richards  and   Woodman's  Air,   Water,  and  Food  from  a  Sanitary  Stand- 
point  8vo,    200 

Rideal's   Disinfection  and  the  Preservation  of  Food 8vo,    400 

Sewage  and  Bacterial  Purification  of  Sewage 8vo,     4  oo 

Soper's  Air  and  Ventilation  of  Subways Large  I2mo,     2  50 

Turneaure  and  Russell's  Public  Water-supplies 8vo,    5  oo 

Venable's  Garbage  Crematories  in  America 8vo,     2  oo 

Method  and  Devices  for  Bacterial  Treatment  of  Sewage 8vo,     3  oo 

Ward  and  Whipple's  Freshwater  Biology 12100,     2  50 

Whipple's  Microscopy  of  Drinking-water 8vo,     3  5  , 

*  Typhod  Fever Large  i2mo,     3  oo 

Value  of  Pure  Water Large  i2mo,     i  oo 

Winslow's  Bacterial  Classification I2mo,     2  50 

Winton's  Microscopy  of  Vegetable  Foods 8vo,    7  50 

MISCELLANEOUS. 

Emmons's  Geological  Guide-book  of  the  Rocky  Mountain  Excursion  of  the 

International  Congress  of  Geologists Large  8vo,  i  50 

FerrePs  Popular  Treatise  on  the  Winds 8vo,  4  oo 

Fitzgerald's  Boston  Machinist i8mo,  i  oo 

Gannett's  Statistical  Abstract  of  the  World t 241110,  75 

Haines's  American  Railway  Management I2mo,  2  50 

*  Hanusek's  The  Microscopy  of  Technical  Products.    (Winton) 8vo.  5  oo 

Owen's  The  Dyeing  and  Cleaning  of  Textile  Fabrics.     (Standage).     (In  Press.) 
Ricketts's  History  of  Rensselaer  Polytechnic  Instituta  1824-1894. 

Large  12  mo, 

Rotherham's  Emphasized  New  Testament , Large  8vo, 

standage's  Decoration  of  Wood,  Glass,  Metal,  etc 12  mo, 

Thome's  Structural  and  Physiological  Botany.    (Bennett) i6mo, 

Westermaier's  Compendium  of  General  Botany.     (Schneider) 8vo, 

Winslow's  Elements  of  Applied  Microscopy I2mo, 


HEBREW  AND  CHALDEE  TEXT-BOOKS. 

Green's  Elementary  Hebrew  Grammar I2mo,    i  25 

Qesenius's  Hebrew  and  Chaldee  Lexicon  to  the  Old  Testament  Scriptures. 

(Tregelles) Small 410 .-half  mor.    5  oo 

19 


THE  LIBRARY 

UNIVERSITY  OF  CALIFORNIA 
LOS  ANGELES 


UNIVERSITY  OF  CALIFORNIA,  LOS  ANGELES 

THE  UNIVERSITY  LIBRARY 
This  book  is  DUE  on  the  last  date  stamped  below 


1951 


T  T 

985      _ 
-R39o"    Compressed  air, 

1909 


TJ 
985 
R39o 
1909 


